Electrolyte Tests

Definition

Electrolytes are positively and negatively charged ions that are found within the cells and extracellular fluids, including blood plasma. A test for electrolytes includes the measurement of sodium, potassium, chloride, and bicarbonate. These ions are needed to assess renal, endocrine, and acid-base function, and are components of both renal function and comprehensive metabolic biochemistry profiles. Other important electrolytes routinely measured in serum or plasma include calcium and phosphorus. These are measured together because they are both affected by bone and parathyroid diseases, and often move in opposing directions. Magnesium is another electrolyte that is routinely measured. Like calcium, it will cause tetany (uncontrolled muscle contractions) when levels are too low in the extracellular fluids.

Purpose

Tests that measure the concentration of electrolytes are needed for both the diagnosis and management of renal, endocrine, acid-base, water balance, and many other conditions. Their importance lies in part with the serious consequences that relatively small changes that diseases or abnormal conditions may cause. For example, the reference range for potassium is 3.6-5.0 mmol/L. Potassium is often a STAT test because values below 3.0 mmol/L are associated with arrhythmia, tachycardia, and cardiac arrest, and values above 6.0 mmol/L are associated with bradycardia and heart failure. Abnormal potassium cannot be treated without reference to bicarbonate, which is a measure of the buffering capacity of the plasma. Sodium bicarbonate and dissolved carbon dioxide act together to resist changes in blood pH. For example, an increased plasma bicarbonate indicates a condition called metabolic alkalosis, which results in a high blood pH level. This may cause hydrogen ions to shift from the cells into the extracellular fluid in exchange for potassium. As potassium moves into the cells, the plasma concentration falls. The low plasma potassium, called hypokalemia, should not be treated by administration of potassium, but by identifying and eliminating the cause of the alkalosis. Administration of potassium would result in hyperkalemia when the acid-base disturbance is corrected. Sodium measurements are very useful in differentiating the cause of an abnormal potassium level.

Conditions such as the overuse of diuretics (drugs that promote lower blood pressure ) often result in low levels of both sodium and potassium. On the other hand, Cushing's disease (adrenocortical hyperfunction) and Addison's disease (adrenocortical insufficiency) drive the sodium and potassium in opposing directions. Chloride levels will follow sodium levels with the exception of acid-base imbalances in which chloride may move in the opposing direction of bicarbonate. In essence, diagnosis and management of a patient with an electrolyte disturbance is best served by measuring all four electrolytes. Use of oral sodium phosphate for bowel cleansing of patients can create electrolyte imbalances. This has lead to recommendations for baseline and post-treatment measurement of sodium, potassium, chloride, bicarbonate, calcium, phosphate, blood urea nitrogen, and creatinine in patients receiving oral sodium phosphate solution.

Description

Sodium is the principal extracellular cation and potassium the principal intracellular cation. Sodium levels are directly related to the osmotic pressure of the plasma. In fact, since an anion is always associated with sodium (usually chloride or bicarbonate), the plasma osmolality (total dissolved solute concentration) can be estimated using the following formula: Osmolality in milliosmoles per killigram water = serum sodium × 2 + Glucose/18 + BUN/2.8 where BUN is the blood urea nitrogen concentration. Since water will often follow sodium by diffusion, loss of sodium leads to dehydration and retention of sodium leads to edema. Conditions that promote increased sodium, called hypernatremia, do so without promoting an equivalent gain in water. Such conditions include diabetes insipidus (water loss by the kidneys ), Cushing's disease, and hyperaldosteronism (increased sodium reabsorption). Many other conditions, such as congestive heart failure, cirrhosis of the liver, and renal disease result in renal retention of sodium, but an equivalent amount of water is retained as well. This results in a condition called total body sodium excess, which causes hypertension and edema, but not an elevated serum sodium concentration. Low serum sodium, called hyponatremia, may result from Addison's disease, excessive diuretic therapy, the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), burns, diarrhea, vomiting, and cystic fibrosis. In fact, the diagnosis of cystic fibrosis is made by demonstrating an elevated chloride concentration (greater than 60 mmol/L) in sweat.

Potassium is the electrolyte used as a hallmark sign of renal failure. Like sodium, potassium is freely filtered by the kidney. However, in the distal tubule sodium is reabsorbed and potassium is secreted. In renal failure, the combination of decreased filtration and decreased secretion combine to cause increased plasma potassium. Hyperkalemia is the most significant and life-threatening complication of renal failure. Hyperkalemia is also commonly caused by hemolytic anemia (release from hemolysed red blood cells), diabetes insipidus, Addison's disease, and digitalis toxicity. Frequent causes of low serum potassium include alkalosis, diarrhea and vomiting, excessive use of thiazide diuretics, Cushing's disease, intravenous fluid administration, and SIADH.

Calcium and phosphorus are measured together because they are both likely to be abnormal in bone and parathyroid disease states. Parathyroid hormone causes resorption of these minerals from bone. However, it promotes intestinal absorption and renal reabsorption of calcium and renal excretion of phosphorus. In hyperparathyroidism, serum calcium will be increased and phosphorus will be decreased. In hypoparathyroidism and renal disease, serum calcium will be low but phosphorus will be high. In vitamin D dependent rickets (VDDR), both calcium and phosphorus will be low; however, calcium is normal while phosphorus is low in vitamin D resistant rickets (VDRR). Differential diagnosis of an abnormal serum calcium is aided by the measurement of ionized calcium (i.e., calcium not bound by protein). Approximately 45% of the calcium in blood is bound to protein, 45% is ionized, and 10% is complexed to anions in the form of undissociated salts. Only the ionized calcium is physiologically active, and the level of ionized calcium is regulated by parathyroid hormone (PTH) via negative feedback (high ionized calcium inhibits secretion of PTH). While hypoparathyroidism, VDDR, renal failure, hypoalbuminemia, hypovitaminosis D, and other conditions may cause low total calcium, only hypoparathyroidism (and alkalosis) will result in low ionized calcium. Conversely, while hyperparathyroidism, malignancies (those that secrete parathyroid hormone-related protein), multiple myeloma, antacids, hyperproteinemia, dehydration, and hypervitaminosis D cause an elevated total calcium, only hyperparathyroidism, malignancy, and acidosis cause an elevated ionized calcium.

Serum magnesium levels may be increased by hemolytic anemia, renal failure, Addison's disease, hyperparathyroidism, and magnesium based antacids. Chronic alcoholism is the most common cause of a low serum magnesium owing to poor nutrition. Serum magnesium is also decreased in diarrhea, hypoparathyroidism, pancreatitis, Cushing's disease, and with excessive diuretic use. Low magnesium can be caused by a number of antibiotics and other drugs and by administration of intravenous solutions. Magnesium is needed for secretion of parathyroid hormone, and therefore, a low serum magnesium can induce hypocalcemia. Magnesium deficiency is very common in regions where the water supply does not contain sufficient magnesium salts. Magnesium acts as a calcium channel blocker, and when cellular magnesium is low, high intracellular calcium results. This leads to hypertension, tachycardia, and tetany. Unfortunately serum total magnesium levels do not correlate well with intracellular magnesium levels, and serum measurement is not very sensitive for detecting chronic deficiency because of compensatory contributions from bone. Ionized magnesium levels are better correlated with intracellular levels because the ionized form can move freely between the cells and extracellular fluids.

Measurement of electrolytes

Electrolytes are measured by a process known as potentiometry. This method measures the voltage that develops between the inner and outer surfaces of an ion selective electrode. The electrode (membrane) is made of a material that is selectively permeable to the ion being measured. For example, sodium electrodes are made from a special glass formula that selectively binds sodium ions. The inside of the electrode is filled with a fluid containing sodium ions, and the outside of the glass membrane is immersed in the sample. A potential difference develops across the glass membrane that is dependent upon the difference in sodium concentration (activity) on the inside and outside of the glass membrane. This potential is measured by comparing it to the potential of a reference electrode. Since the potential of the reference electrode is held constant, the difference in voltage between the two electrodes is attributed to the concentration of sodium in the sample. Ion selective membranes can be made from materials other than glass. For example, the antibiotic valinomycin is used to make potassium-measuring electrodes. Neutral carrier ionophores selective for lithium, calcium, and magnesium are also used for measurement of these substances in laboratory medicine. Ion selective electrodes can be used to measure whole blood, serum, or plasma since they respond to the electrolyte activity in the water phase of the sample only. One important aspect of electrolyte measurement is an artifact (erroneous result) called pseudohyponatremia that may occur when sodium is measured using a diluted blood sample. This happens when the plasma contains excessively high lipids or protein. These solids displace plasma water from the specimen, resulting in a low measurement of sodium that does not occur with an undiluted sample.

Total calcium and magnesium are usually measured by colorimetric procedures called dye binding assays. Calcium is displaced from protein by dilute acid or alkali and reacts with a dye (arsenazo III or cresolphthalein complexone) to form a colored product. When crosolphthalein complexone is used, 8-hydroxyquinoline is added to bind magnesium, which also reacts with this dye. Magnesium is commonly measured by its reaction with a dye called Calmagite. A calcium chelator such as EGTA is added to prevent interference from calcium. Both calcium and magnesium may be measured by atomic absorption spectrophotometry. This procedure is more complex than colorimetric methods, but is also more accurate. Phosphorus is measured by reacting it with ammonium molybdate at an acid pH. The rate of ammonium phosphomolybdate formation is measured at 340 nm and is proportional to the inorganic phosphorus concentration (mono- and dihydrogen phosphate) of the sample.

Precautions

Electrolyte tests are performed on heparinized whole blood, heparinized plasma, or serum, usually collected from a vein or capillary. Venipuncture is performed observing universal precautions for the prevention of transmission of bloodborne pathogens. In order to prevent hemoconcentration, the tourniquet must be removed from the arm as soon as the blood starts to flow. The needle gauge must be sufficient in width to prevent mechanical damage to the red blood cells that will result in hemolysis (rupture of the membrane of the red blood cells). Because the concentration of potassium, magnesium, and phosphorus within red blood cells is much higher than in the plasma, hemolysis will cause falsely elevated results for these analytes. Plasma is often preferred over serum for measuring potassium, as the process of blood clotting can release potassium from platelets. Heparin is the only anticoagulant acceptable for electrolyte testing, as all other anticoagulants act by chelating calcium. Samples for ionized calcium should be collected using balanced (low) heparin which has a concentration of 20 U/mL. Higher concentrations bind calcium. Ionized calcium samples should be transported and stored on ice under anaerobic conditions and measured within 30 minutes of sample collection as pH changes in the blood will affect the ionized calcium.

Special procedures are followed when collecting a sweat sample for electrolyte analysis. This procedure, called pilocarpine iontophoresis, uses electric current applied to the arm of the patient (usually an infant) in order to convey the pilocarpine to the sweat glands where it will stimulate sweating. Care must be taken to ensure that the collection device (macroduct tubing or gauze) does not become contaminated and that the patient's parent or guardian understands the need for the electrical equipment employed.

Preparation

Usually no special preparation is necessary by the patient. Samples for calcium and phosphorus and for magnesium should be collected following an eight-hour fast.

Aftercare

Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Applying warm packs to the puncture site relieves discomfort.

Complications

Minor temporary discomfort may occur with any blood test, but there are no complications specific to electrolyte testing.

Results

Electrolyte concentrations are similar whether measured in serum or plasma. Values are expressed as mmol/L for sodium, potassium, chloride, and bicarbonate. Magnesium results are often reported as milliequivalents per liter (meq/L) or in mg/dL. Total calcium is usually reported in mg/dL and ionized calcium in mmol/L. Since severe electrolyte disturbances can be associated with life-threatening consequences such as heart failure, shock, coma, or tetany alert values are used to warn physicians of impending crisis. Typical reference ranges and alert values are cited below.

KEY TERMS

Anion— A negatively charged ion.

Cation— A positively charged ion.

Tetany— Inappropriately sustained muscle spasms.

• serum or plasma sodium: 135-145 mmol/L; Alert levels: less than 120 mmol/L and greater than 160 mmol/L

• serum potassium: 3.6-5.4 mmol/L (plasma, 3.6-5.0mmol/L); Alert levels: less than 3.0 mmol/L and greater than 6.0 mmol/L
• serum or plasma chloride: 98-108 mmol/L
• sweat chloride: 4-60 mmol/L
• serum or plasma bicarbonate: 18-24 mmol/L (as total carbon dioxide, 22-26 mmol/L); Alert levels: less than 10 mmol/L and greater than 40 mmol/L
• serum calcium: 8.5-10.5 mg/dL (2.0-2.5 mmol/L); Alert levels: less than 6.0 mg/dL and greater than 13.0 mg/dL
• ionized calcium: 1.0-1.3 mmol/L
• serum inorganic phosphorus: 2.3-4.7 mg/dL (children, 4.0-7.0 mg/dL); Alert level: less than 1.0 mg/dL
• serum magnesium: 1.8-3.0 mg/dL (1.2-2.0 meq/L or 0.5-1.0 mmol/L)
• ionized magnesium: 0.53-0.67 mmol/L
• osmolality (calculated) 280-300 mosm/Kg

Health care team roles

A physician orders electrolyte tests and interprets the results. A nurse or phlebotomist usually collects the blood sample by venipuncture. In some instances the nurse performs the electrolyte test using a point-of-care instrument consisting of a single use cartridge of ion-selective electrodes and a battery operated analyzer. In the laboratory setting electrolyte tests are performed by clinical laboratory scientists/medical technologists or clinical laboratory technicians/medical laboratory technicians. Nurses, nurse practitioners, and physician assistants may find themselves involved in explaining results to patients and advising them regarding treatment or dietary correction of any problems identified.

Resources

BOOKS

Fishbach, Frances Talaska. A Manual of Laboratory and Diagnostic Tests, 6th ed. Lippincott, 2003.

Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2006. 45th ed. Lange Medical Books/McGraw-Hill, 2006.

PERIODICALS

Kubetin, Sally Koch. "FDA Alert on Sodium Phosphate." Internal Medicine News (Jan. 15, 2002):23.

OTHER

MedLine Plus. "Electrolytes." 2001. 〈http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm〉.

Electrolyte Tests

Definition

Electrolytes are positively and negatively charged molecules, called ions, that are found within cells, between cells, in the bloodstream, and in other fluids throughout the body. Electrolytes with a positive charge include sodium, potassium, calcium, and magnesium; the negative ions are chloride, bicarbonate, and phosphate. The concentrations of these ions in the bloodstream remain fairly constant throughout the day in a healthy person. Changes in the concentration of one or more of these ions can occur during various acute and chronic disease states and can lead to serious consequences.

Purpose

Tests that measure the concentration of electrolytes are useful in the emergency room and to obtain clues for the diagnosis of specific diseases. Electrolyte tests are used for diagnosing dietary deficiencies, excess loss of nutrients due to urination, vomiting, and diarrhea, or abnormal shifts in the location of an electrolyte within the body. When an abnormal electrolyte value is detected, the physician may either act to immediately correct the imbalance directly (in the case of an emergency) or run further tests to determine the underlying cause of the abnormal electrolyte value. Electrolyte disturbances can occur with malfunctioning of the kidney (renal failure), infections that produce severe and continual diarrhea or vomiting, drugs that cause loss of electrolytes in the urine (diuretics), poisoning due to accidental consumption of electrolytes, or diseases involving hormones that regulate electrolyte concentrations.

Precautions

Electrolyte tests are performed from routine blood tests. The techniques are simple, automated, and fairly uniform throughout the United States. During the preparation of blood plasma or serum, health workers must take care not to break the red blood cells, especially when testing for serum potassium. Because the concentration of potassium within red blood cells is much higher than in the surrounding plasma or serum, broken cells would cause falsely elevated potassium levels.

Description

Electrolyte tests are typically conducted on blood plasma or serum, urine, and diarrheal fluids. Electrolytes can be classified in at least five different ways. One way is that some electrolytes tend to exist mostly inside cells, or are intracellular, while others tend to be outside cells, or are extracellular. Potassium, phosphate, and magnesium occur at much greater levels inside the cell than outside, while sodium and chloride occur at much greater levels extracellularly. A second classification distinguishes those electrolytes that participate directly in the transmission of nerve impulses and those that do not. Sodium, potassium, and calcium are the important electrolytes involved in nerve impulses, and disorders affecting them are most closely associated with neurological disorders. A third classification focuses on electrolytes that are able to form a tight union, or complex, with one another. Calcium and phosphate have the greatest tendency to form complexes with each other. Disorders that cause an increase in either plasma calcium or phosphate can result in the deposit of calcium-phosphate crystals in the soft tissues of the body. A fourth classification concerns those electrolytes that influence the acidity or alkalinity of the bloodstream, also known as the pH. The pH of the bloodstream is normally in the range of 7.35-7.45. A decrease below this range is called acidosis, while a pH above this range is called alkalosis. The electrolytes most closely associated with the pH of the bloodstream are bicarbonate, chloride, and phosphate.

Preparation

All electrolyte tests can be performed on plasma or serum. Plasma is prepared by withdrawing a blood sample and placing it in a test tube containing a chemical that prevents blood from clotting (an anticoagulant). Serum is prepared by withdrawing a blood sample, placing it in a test tube, and allowing it to clot. The blood spontaneously clots within a minute of withdrawing the blood from a vein. The serum or plasma is then rapidly spun with a centrifuge in order to remove the blood cells or clot.

Normal results

Electrolyte concentrations are similar whether measured in serum or plasma. Values can be expressed in terms of weight per unit volume (mg/ deciliter; mg/dL) or in the number of molecules in a volume, or molarity (moles or millimoles/liter; M or mM). The range of normal values sometimes varies slightly between different age groups, for males and females, and between different analytical laboratories.

The normal level of serum sodium is in the range of 136-145 mM. The normal levels of serum potassium are 3.5-5.0 mM. Note that sodium occurs at a much higher concentration than potassium. The normal concentration of total serum calcium (bound calcium plus free calcium) is in the range of 8.8-10.4 mg/dL. About 40% of the total calcium in the plasma is loosely bound to proteins; this calcium is referred to as bound calcium. The normal range of free calcium is 4.8-5.2 mg/dL. The normal concentration of serum magnesium is in the range of 2.0-3.0 mg/dL.

The normal concentration range of chloride is 350-375 mg/dL or 98-106 mM. The normal level of phosphate, as expressed as the concentration of phosphorus, is 2.0-4.3 mg/dL. Bicarbonate is an electrolyte that is freely and spontaneously interconvertable with carbonic acid and carbon dioxide. The normal concentration of carbonic acid (H₂CO₃) is about 1.35 mM. The normal concentration of bicarbonate (HCO₃⁻) is about 27 mM. The concentration of total carbon dioxide is the sum of carbonic acid and bicarbonate; this sum is normally in the range of 26-28 mM. The ratio of bicarbonate/carbonic acid is more significant than the actual concentrations of these two forms of carbon dioxide. Its normal value is 27/1.35 (equivalent to 20/1).

Abnormal results

Positively charged electrolytes

High serum sodium levels (hypernatremia ) occur at sodium concentrations over 145 mM, with severe hypernatremia over 152 mM. Hypernatremia is usually caused by diseases that cause excessive urination. In these cases, water is lost, but sodium is still retained in the body. The symptoms include confusion and can lead to convulsions and coma. Low serum sodium levels (hyponatremia ) are below 130 mM, with severe hyponatremia at or below 125 mM. Hyponatremia often occurs with severe diarrhea, with losses of both water and sodium, but with sodium loss exceeding water loss. Hyponatremia provokes clinical problems only if serum sodium falls below 125 mM, especially if this has occurred rapidly. The symptoms can be as mild as tiredness but may lead to convulsions and coma.

High serum potassium (hyperkalemia ) occurs at potassium levels above 5.0 mM; it is considered severe over 8.0 mM. Hyperkalemia is relatively uncommon, but sometimes occurs in patients with kidney failure who take potassium supplements. Hyperkalemia can result in abnormal beating of the heart (cardiac arrhythmias ). Low serum potassium (hypokalemia ) occurs when serum potassium falls below 3.0 mM. It can result from low dietary potassium, as during starvation or in patients with anorexia nervosa; from excessive losses via the kidneys, as caused by diuretic drugs; or by diseases of the adrenal or pituitary glands. Mild hypokalemia causes muscle weakness, while severe hypokalemia can cause paralysis, the inability to breathe, and cardiac arrhythmias.

High levels of calcium ions (hypercalcemia ) occur at free calcium ion concentrations over 5.2 mg/dL or total serum calcium above 10.4 mg/dL. Hypercalcemia usually occurs when the body dissolves bone at an abnormally fast rate, increasing both serum calcium and serum phosphate. Sudden hypercalcemia can cause vomiting and coma, while prolonged and moderate hypercalcemia results in the deposit of calcium phosphate crystals in the kidneys and eye. Hypocalcemia occurs when serum free calcium ions fall below 4.4 mg/dL, or when total serum calcium falls below 8.8 mg/dL. Hypocalcemia can result from hypoparathyroidism (low parathyroid hormone), from failure to produce 1,25-dihydroxyvitamin D, from low levels of plasma magnesium, and from phosphate poisoning (the phosphate enters the bloodstream and forms a complex with the free serum calcium). Hypocalcemia can cause depression and muscle spasms.

Hypermagnesemia occurs at serum magnesium levels over 25 mM (60 mg/dL). Hypermagnesemia is rare but can occur with the excessive consumption of magnesium salts. Hypomagnesemia occurs when serum magnesium levels fall below 0.8 mM, and can result from poor nutrition. Chronic alcoholism is the most common cause of hypomagnesemia, in part because of poor diet. Magnesium levels below 0.5 mM (1.2 mg/dL) cause serum calcium levels to decline. Some of the symptoms of hypomagnesemia, including twitching and convulsions, actually result from the concurrent hypocalcemia. Hypomagnesemia can also result in hypokalemia and thereby cause cardiac arrhythmias.

Negatively charged electrolytes

Serum chloride levels sometimes increase to abnormal levels as an undesirable side effect of medical treatment with sodium chloride or ammonium chloride. The toxicity of chloride results not from the chloride itself, but from the fact that the chloride occurs as the acid, hydrogen chloride (more commonly known as hydrochloric acid, or HCl). An overdose of chloride may cause the accumulation of hydrochloric acid in the bloodstream, with consequent acidosis. Renal tubular acidosis, one of many kidney diseases, involves the failure to release acid into the urine. The acidosis produces weakness, headache, nausea, and cardiac arrest. Low plasma chloride leads to the opposite situation: a decline in the acid content of the bloodstream. This is known as alkalization of the bloodstream, or alkalosis. Hydrochloric acid, originally from extracellular fluids, can be lost by vomiting. At its most severe, alkalosis results in paralysis (tetany).

Hyperphosphatemia occurs at serum phosphate levels above 5 mg/dL. It can result from the failure of the kidneys to excrete phosphate into the urine, causing phosphate to accumulate in the bloodstream. Hyperphosphatemia can also be caused by the impaired action of parathyroid hormone and by phosphate poisoning. Severe hyperphosphatemia can cause paralysis, convulsions, and cardiac arrest. These symptoms result because the phosphate, occurring in elevated levels, complexes with free serum calcium, resulting in hypocalcemia. Tests for heart function (an electrocardiogram) and parathyroid hormone levels are used in the diagnosis of hyperphosphatemia. Hypophosphatemia occurs if serum phosphorus falls to 2.0 mg/dL or lower. It often results from a shift of inorganic phosphate from the bloodstream to various organs and tissues. This shift can be caused by a rise in pH (alkalization) of the bloodstream, which can occur during hyperventilation, a reaction in various disease states. A shift in phosphate to intracellular tissues may draw calcium away from the bloodstream via the formation of insoluble calcium phosphate crystals within cells, with consequent hypocalcemia. Thus, tests for abnormalities in phosphate metabolism also involve tests for serum calcium.

Bicarbonate metabolism involves several compounds. When dietary starches, sugars, and fats are broken down for energy production, carbon dioxide is created. Much of this carbon dioxide (CO₂) spontaneously converts to carbonic acid (H₂CO₃), and some of the carbonic acid spontaneously converts to bicarbonate (HCO₃⁻) plus a hydrogen ion (H⁻). Eventually, almost every molecule of carbon dioxide produced in the body, whether in the form of carbon dioxide, carbonic acid, or bicarbonate, must convert back to carbon dioxide in order to leave via the lungs during normal breathing.

If one holds one's breath, carbon dioxide cannot escape from the lungs, but continues to be generated within the body. This results in an increase in production of carbonic acid. A portion of the carbonic acid breaks apart (dissociates), causing an increase in hydrogen ions in the plasma, with a resulting acidosis. Tests for serum bicarbonate levels are accompanied by tests for acidosis (pH test). Conversely, when one breathes too rapidly (hyperventilation), the carbon dioxide is drawn off from the bloodstream and expelled in the breath at an increased rate. This results in an increase in the rate of combination of bicarbonate with hydrogen ions, resulting in alkalosis. Acidosis and alkalosis can be produced by means other than by altering the rate of breathing. The carbonic acid and bicarbonate in the bloodstream minimize (or buffer) any trend to acidosis or alkalosis. Tests for bicarbonate are generally accompanied by tests for blood pH and possibly tests for kidney malfunction, abnormal hormone function, or gastrointestinal disorders.

Resources

PERIODICALS

Fried, L. F., and P. M. Palevsky. "Hyponatremia and hypernatremia." Medical Clinics of North America 81 (1997): 585-609.

Electrolyte tests

Definition

Electrolytes are positively and negatively charged molecules called ions, that are found within the body's cells and extracellular fluids, including blood plasma. A test for electrolytes includes the measurement of sodium, potassium, chloride, and bicarbonate. These ions are measured to assess renal (kidney), endocrine (glandular), and acid-base function, and are components of both renal function and comprehensive metabolic biochemistry profiles. Other important electrolytes routinely measured in serum or plasma include calcium and phosphorus. These are measured together because they are both affected by bone and parathyroid diseases, and often move in opposing directions. Magnesium is another electrolyte that is routinely measured. Like calcium, it will cause tetany (uncontrolled muscle contractions) when levels are too low in the extracellular fluids.

Purpose

Tests that measure the concentration of electrolytes are needed for both the diagnosis and management of renal, endocrine, acid-base, water balance, and many other conditions. Their importance lies in part with the serious consequences that follow from the relatively small changes that diseases or abnormal conditions may cause. For example, the reference range for potassium is 3.6-5.0 mmol/l. Potassium is often a STAT (needed immediately) test because values below 3.0 mmol/l are associated with arrhythmia (irregular heartbeat), tachycardia (rapid heartbeat), and cardiac arrest, and values above 6.0 mmol/L are associated with bradycardia (slow heartbeat) and heart failure. Abnormal potassium cannot be treated without reference to bicarbonate, which is a measure of the buffering capacity of the plasma. Sodium bicarbonate and dissolved carbon dioxide act together to resist changes in blood pH. For example, an increased plasma bicarbonate indicates a condition called metabolic alkalosis, which results in blood pH that is too high. This may cause hydrogen ions to shift from the cells into the extracellular fluid in exchange for potassium. As potassium moves into the cells, the plasma concentration falls. The low plasma potassium, called hypokalemia, should not be treated by administration of potassium, but by identifying and eliminating the cause of the alkalosis. Administration of potassium would result in hyperkalemia when the acid-base disturbance is corrected. Sodium measurements are very useful in differentiating the cause of an abnormal potassium result. Conditions such as the overuse of diuretics (drugs that promote lower blood pressure) often result in low levels of both sodium and potassium. On the other hand, Cushing's disease (adrenocortical over-activity) and Addison's disease (adrenocortical under-activity) drive the sodium and potassium in opposing directions. Chloride levels will follow sodium levels except in the case of acid-base imbalances, in which chloride may move in the opposing direction of bicarbonate. In short, diagnosis and management of a patient with an electrolyte disturbance is best served by measuring all four electrolytes.

Description

Sodium is the principal extracellular cation and potassium the principal intracellular cation. A cation is an ion with a positive charge. An anion is an ion with a negative charge. Sodium levels are directly related to the osmotic pressure of the plasma. In fact, since an anion is always associated with sodium (usually chloride or bicarbonate), the plasma osmolality (total dissolved solute concentration) can be estimated. Since water will often follow sodium by diffusion, loss of sodium leads to dehydration and retention of sodium leads to edema. Conditions that promote increased sodium, called hypernatremia, do so without promoting an equivalent gain in water. Such conditions include diabetes insipidus (water loss by the kidneys), Cushing's disease, and hyperaldosteronism (increased sodium reabsorption). Many other conditions, such as congestive heart failure, cirrhosis of the liver, and renal disease result in renal retention of sodium, but an equivalent amount of water is retained as well. This results in a condition called total body sodium excess, which causes hypertension and edema, but not an elevated serum sodium concentration. Low serum sodium, called hyponatremia, may result from Addison's disease, excessive diuretic therapy, the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), burns, diarrhea, vomiting, and cystic fibrosis. In fact, the diagnosis of cystic fibrosis is made by demonstrating an elevated chloride concentration (greater than 60 mmol/l) in sweat.

Potassium is the electrolyte used as a hallmark sign of renal failure. Like sodium, potassium is freely filtered by the kidney. However, in the distal tubule sodium is reabsorbed and potassium is secreted. In renal failure, the combination of decreased filtration and decreased secretion combine to cause increased plasma potassium. Hyperkalemia is the most significant and life-threatening complication of renal failure. Hyperkalemia is also commonly caused by hemolytic anemia (release from hemolysed red blood cells), diabetes insipidus, Addison's disease, and digitalis toxicity. Frequent causes of low serum potassium include alkalosis, diarrhea and vomiting, excessive use of thiazide diuretics, Cushing's disease, intravenous fluid administration, and SIADH.

Calcium and phosphorus are measured together because they are both likely to be abnormal in bone and parathyroid disease states. Parathyroid hormone causes resorption of these minerals from bone. However, it promotes intestinal absorption and renal reabsorption of calcium and renal excretion of phosphorus. In hyperparathyroidism, serum calcium will be increased and phosphorus will be decreased. In hypoparathyroidism and renal disease, serum calcium will be low but phosphorus will be high. In vitamin D dependent rickets (VDDR), both calcium and phosphorus will be low; however, calcium is normal while phosphorus is low in vitamin D resistant rickets (VDRR). Differential diagnosis of an abnormal serum calcium is aided by the measurement of ionized calcium (i.e., calcium not bound by protein). Approximately 45% of the calcium in blood is bound to protein, 45% is ionized, and 10% is complexed to anions in the form of undissociated salts. Only the ionized calcium is physiologically active, and the level of ionized calcium is regulated by parathyroid hormone (PTH) via negative feedback (high ionized calcium inhibits secretion of PTH). While hypoparathyroidism, VDDR, renal failure, hypoalbuminemia, hypovitaminosis D, and other conditions may cause low total calcium, only hypoparathyroidism (and alkalosis) will result in low ionized calcium. Conversely, while hyperparathyroidism, malignancies (those that secrete parathyroid hormone-related protein), multiple myeloma, antacids, hyperproteinemia, dehydration, and hypervitaminosis D cause an elevated total calcium, only hyperparathyroidism, malignancy, and acidosis cause an elevated ionized calcium.

Serum magnesium levels may be increased by hemolytic anemia, renal failure, Addison's disease, hyperparathyroidism, and magnesium-based antacids. Chronic alcoholism is the most common cause of a low serum magnesium owing to poor nutrition. Serum magnesium is also decreased in diarrhea, hypoparathyroidism, pancreatitis, Cushing's disease, and with excessive diuretic use. Low magnesium can be caused by a number of antibiotics and other drugs and by administration of intravenous solutions. Magnesium is needed for secretion of parathyroid hormone, and therefore, a low serum magnesium can induce hypocalcemia. Magnesium deficiency is very common in regions where the water supply does not contain sufficient magnesium salts. Magnesium acts as a calcium channel blocker, and when cellular magnesium is low, high intracellular calcium results. This leads to hypertension, tachycardia, and tetany. Unfortunately serum total magnesium levels do not correlate well with intracellular magnesium levels, and serum measurement is not very sensitive for detecting chronic deficiency because of compensatory contributions from bone. Ionized magnesium levels are better correlated with intracellular levels because the ionized form can move freely between the cells and extracellular fluids.

Measurement of electrolytes

Electrolytes are measured by a process known as potentiometry. This method measures the voltage that develops between the inner and outer surfaces of an ion selective electrode. The electrode (membrane) is made of a material that is selectively permeable to the ion being measured. This potential is measured by comparing it to the potential of a reference electrode. Since the potential of the reference electrode is held constant, the difference in voltage between the two electrodes is attributed to the concentration of ion in the sample.

Precautions

Electrolyte tests are performed on whole blood, plasma, or serum, usually collected from a vein or capillary.

Special procedures are followed when collecting a sweat sample for electrolyte analysis. This procedure, called pilocarpine iontophoresis, uses electric current applied to the arm of the patient (usually an infant) in order to convey the pilocarpine to the sweat glands where it will stimulate sweating. Care must be taken to ensure that the collection device (macroduct tubing or gauze) does not become contaminated and that the patient's parent or guardian understands the need for the electrical equipment employed.

Preparation

Usually no special preparation is necessary by the patient. Samples for calcium and phosphorus and for magnesium should be collected following an eight-hour fast.

Aftercare

Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Applying warm packs to the puncture site relieves discomfort.

Risks

Minor temporary discomfort may occur with any blood test, but there are no complications specific to electrolyte testing.

Normal results

Electrolyte concentrations are similar whether measured in serum or plasma. Values are expressed as mmol/L for sodium, potassium, chloride, and bicarbonate. Magnesium results are often reported as milliequivalents per liter (meq/L) or in mg/dL. Total calcium is usually reported in mg/dL and ionized calcium in mmol/L. Since severe electrolyte disturbances can be associated with life-threatening consequences such as heart failure, shock, coma, or tetany, alert values are used to warn physicians of impending crisis. Typical reference ranges and alert values are cited below:

• serum or plasma sodium: 135–145 mmol/l; alert levels: less than 120 mmol/l and greater than 160 mmol/l
• serum potassium: 3.6–5.4 mmol/l (plasma, 3.6–5.0 mmol/l); alert levels: less than 3.0 mmol/l and greater than 6.0 mmol/l
• serum or plasma chloride: 98–108 mmol/l
• sweat chloride: 4–60 mmol/l
• serum or plasma bicarbonate: 18–24 mmol/l (as total carbon dioxide, 22–26 mmol/l); alert levels: less than 10 mmol/l and greater than 40 mmol/l
• serum calcium: 8.5–10.5 mg/dl (2.0–2.5 mmol/l); alert levels: less than 6.0 mg/dl and greater than 13.0 mg/dl
• ionized calcium: 1.0–1.3 mmol/l
• serum inorganic phosphorus: 2.3–4.7 mg/dl (children, 4.0–7.0 mg/dl); alert level: less than 1.0 mg/dl
• serum magnesium: 1.8–3.0 mg/dl (1.2–2.0 meq/l or 0.5–1.0 mmol/l)
• ionized magnesium: 0.53–0.67 mmol/l
• osmolality (calculated) 280–300 mosm/kg

Resources

books

henry, j. b. clinical diagnosis and management of laboratory methods. 20th ed. philadelphia: w. b. saunders company, 2001.

tierney, lawrence m., stephen j. mcphee, and maxine a. papadakis. current medical diagnosis and treatment 2001. 40th ed. new york: lange medical books/mcgraw-hill, 2001.

wallach, jacques. interpretation of diagnostic tests. 7th ed. philadelphia: lippincott williams & wilkins, 2000.

other

medline plus. "electrolytes." october 18, 2001 [cited april 4, 2003]. <http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm>.

national institutes of health. [cited april 5, 2003]. <http://www.nlm.nih.gov/medlineplus/encyclopedia.html>.

Erika J. Norris Mark A. Best, M.D.

Electrolyte Tests

Definition
Purpose
Description
Precautions
Preparation
Aftercare
Risks
Normal results

Definition

Electrolytes are positively and negatively charged molecules, called ions, that are found within the body’s cells and extracellular fluids, including blood plasma. A test for electrolytes includes the measurement of sodium, potassium, chloride, and bicarbonate. These ions are measured to assess renal (kidney), endocrine (glandular), and acid-base function, and are components of both renal function and comprehensive metabolic biochemistry profiles. Other important electrolytes routinely measured in serum or plasma include calcium and phosphorus. These are measured together because they are both affected by bone and parathyroid diseases, and often move in opposing directions. Magnesium is another electrolyte that is routinely measured. Like calcium, it will cause tetany (uncontrolled muscle contractions) when levels are too low in the extracellular fluids.

Purpose

Tests that measure the concentration of electrolytes are needed for both the diagnosis and management of renal, endocrine, acid-base, water balance, and many other conditions. Their importance lies in part with the serious consequences that follow from the relatively small changes that diseases or abnormal conditions may cause. In short, diagnosis and management of a patient with an electrolyte disturbance is best served by measuring all four electrolytes.

Description

Sodium levels are directly related to the water concentration in blood plasma. Since water will often follow sodium, loss of sodium leads to dehydration, and retention of sodium leads to edema (swelling, water retention). Conditions that promote increased sodium, called hypernatremia, do so without promoting an equivalent gain in water. Such conditions include diabetes insipidus (water loss by the kidneys), Cushing’s disease, and hyperaldosteronism (increased sodium reabsorption).

Many other conditions, such as congestive heart failure, cirrhosis of the liver, and renal disease result in renal retention of sodium, but an equivalent amount of water is retained as well. This results in a condition called total body sodium excess, which causes hypertension and edema, but not an elevated serum sodium concentration. Low serum sodium, called hyponatremia, may result from Addison’s disease, excessive diuretic therapy, the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), burns, diarrhea, vomiting, and cystic fibrosis. In fact, the diagnosis of cystic fibrosis is made by demonstrating an elevated chloride concentration (greater than 60 mmol/L) in sw

Potassium is the electrolyte used as a hallmark sign of renal failure. Like sodium, potassium is freely filtered by the kidney. In renal failure, the combination of decreased filtration and decreased secretion combine to cause increased plasma potassium. Hyper-kalemia is the most significant and life-threatening complication of renal failure. Hyperkalemia is also commonly caused by hemolytic anemia (release from hemolysed red blood cells), diabetes insipidus, Addison’s disease, and digitalis toxicity. Frequent causes of low serum potassium include alkalosis, diarrhea and vomiting, excessive use of thiazide diuretics, Cushing’s disease, intravenous fluid administration, and SIADH.

The reference range for potassium is 3.6-5.0 mmol/L (or mEq/L). Potassium is often a STAT (needed immediately) test because values below 3.0 mmol/L (or mEq/L) are associated with arrhythmia (irregular heartbeat), tachycardia (rapid heartbeat), and cardiac arrest, and values above 6.0 mmol/L (or mEq/L) are associated with bradycardia (slow heartbeat) and heart failure. Abnormal potassium cannot be treated without reference to bicarbonate, which is a measure of the buffering capacity of the plasma. Sodium bicarbonate and dissolved carbon dioxide act together to resist changes in blood pH. For example, an increased plasma bicarbonate indicates a condition called metabolic alkalosis, which results in blood pH that is too high. This may cause hydrogen ions to shift from the cells into the extracellular fluid in exchange for potassium. As potassium moves into the cells, the plasma concentration falls. The low plasma potassium, called hypokalemia, should not be treated by administration of potassium, but by identifying and eliminating the cause of the alkalosis. Administration of potassium would result in hyperkalemia (too much potassium) when the acid-base disturbance is corrected.

Sodium measurements are very useful in differentiating the cause of an abnormal potassium result. Conditions such as the overuse of diuretics (drugs that promote lower blood pressure) often result in low levels of both sodium and potassium. On the other hand, Cushing’s disease (adrenocortical over-activity) and Addison’s disease (adrenocortical under-activity) drive the sodium and potassium in opposing directions. Chloride levels will follow sodium levels except in the case of acid-base imbalances, in which chloride may move in the opposing direction of bicarbonate.

Calcium and phosphorus are measured together because they are both likely to be abnormal in bone and parathyroid disease states. Parathyroid hormone causes resorption of these minerals from bone. However, it promotes intestinal absorption and renal reab-sorption of calcium, and renal excretion of phosphorus. In hyperparathyroidism, serum calcium will be increased and phosphorus will be decreased. In hypoparathyroidism and renal disease, serum calcium will be low but phosphorus will be high. In vitamin D dependent rickets (VDDR), both calcium and phosphorus will be low; however, calcium is normal while phosphorus is low in vitamin D resistant rickets (VDRR).

Differential diagnosis of an abnormal serum calcium is aided by the measurement of ionized calcium (i.e., calcium not bound by protein). Only the ionized calcium is physiologically active, and the level of ionized calcium is regulated by parathyroid hormone (PTH) via negative feedback (high ionized calcium inhibits secretion of PTH). While hypoparathyroid-ism, VDDR, renal failure, hypoalbuminemia, hypovi-taminosis D, and other conditions may cause low total calcium, only hypoparathyroidism (and alkalosis) will result in low ionized calcium. Conversely, while hyperparathyroidism, malignancies (those that secrete parathyroid hormone-related protein), multiple myeloma, antacids, hyperproteinemia, dehydration, and hyper-vitaminosis D cause an elevated total calcium, only hyperparathyroidism, malignancy, and acidosis cause an elevated ionized calcium.

Serum magnesium levels may be increased by hemolytic anemia, renal failure, Addison’s disease, hyperparathyroidism, and magnesium-based antacids. Chronic alcoholism is the most common cause of a low serum magnesium owing to poor nutrition. Serum magnesium is also decreased in

KEY TERMS

Arrhythmia— Abnormal rhythm of the heartbeat.

Bradycardia— Slow heart beat, usually under 60 beats per minute.

Hyperkalemia— An abnormally high concentration of potassium in the blood.

Hypocalcemia— An abnormally small concentration of calcium in the blood.

Hypokalemia— An abnormally small concentration of potassium in the blood.

Tachycardia— Rapid heart beat, generally over 100 beats per minute.

Tetany— Inappropriately sustained muscle spasms.

diarrhea, hypoparathyroidism, pancreatitis, Cushing’s disease, and with excessive diuretic use. Low magnesium can be caused by a number of antibiotics and other drugs and by administration of intravenous solutions. Magnesium is needed for secretion of parathyroid hormone, and therefore, a low serum magnesium can induce hypocalcemia (too little calcium). Magnesium deficiency is very common in regions where the water supply does not contain sufficient magnesium salts. Magnesium acts as a calcium channel blocker, and when cellular magnesium is low, high intracellular calcium results. This leads to hypertension, tachycardia, and tetany.

Measurement of electrolytes

Electrolytes are measured by a process known as potentiometry. This method measures the voltage that develops between the inner and outer surfaces of an ion selective electrode. The electrode (membrane) is made of a material that is selectively permeable to the ion being measured. This potential is measured by comparing it to the potential of a reference electrode. Since the potential of the reference electrode is held constant, the difference in voltage between the two electrodes is attributed to the concentration of ion in the sample.

Precautions

Electrolyte tests are performed on whole blood, plasma, or serum, usually collected from a vein or capillary.

Special procedures are followed when collecting a sweat sample for electrolyte analysis. This procedure, called pilocarpine iontophoresis, uses electric current applied to the arm of the patient (usually an infant) in order to convey the pilocarpine to the sweat glands where it will stimulate sweating. Care must be taken to ensure that the collection device (macroduct tubing or gauze) does not become contaminated and that the patient’s parent or guardian understands the need for the electrical equipment employed.

Preparation

Usually no special preparation is necessary by the patient. Samples for calcium and phosphorus and for magnesium should be collected following an eight-hour fast.

Aftercare

Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Applying warm packs to the puncture site relieves discomfort.

Risks

Minor temporary discomfort may occur with any blood test, but there are no complications specific to electrolyte testing.

Normal results

Electrolyte concentrations are similar whether measured in serum or plasma. Values are expressed as mmol/L for sodium, potassium, chloride, and bicarbonate. Magnesium results are often reported as milliequivalents per liter (meq/L) or in mg/dL. Total calcium is usually reported in mg/dL and ionized calcium in mmol/ L. Since severe electrolyte disturbances can be associated with life-threatening consequences such as heart failure, shock, coma, or tetany, alert values are used to warn physicians of impending crisis. Typical reference ranges and alert values are cited below:

• serum or plasma sodium: 135-145 mmol/L; alert levels: less than 120 mmol/L and greater than 160 mmol/L
• serum potassium: 3.6-5.4 mmol/L (plasma, 3.6-5.0 mmol/L); alert levels: less than 3.0 mmol/L and greater than 6.0 mmol/L
• serum or plasma chloride: 98-108 mmol/L
• sweat chloride: 4-60 mmol/L
• serum or plasma bicarbonate: 18-24 mmol/L (as total carbon dioxide, 22–26 mmol/L); alert levels: less than 10 mmol/L and greater than 40 mmol/L
• serum calcium: 8.5-10.5 mg/dL (2.0-2.5 mmol/L); alert levels: less than 6.0 mg/dL and greater than 13.0 mg/dL
• ionized calcium: 1.0-1.3 mmol/L
• serum inorganic phosphorus: 2.3–4.7 mg/dL (children, 4.0–7.0 mg/dL); alert level: less than 1.0 mg/dL
• serum magnesium: 1.8-3.0 mg/dL (1.2-2.0 meq/L or 0.5-1.0 mmol/L)
• ionized magnesium: 0.53-0.67 mmol/L
• osmolality (calculated) 280-300 mosm/Kg

Resources

BOOKS

Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2008. 47th ed. New York: Lange Medical Books/ McGraw-Hill, 2007.

Wallach, Jacques. Interpretation of Diagnostic Tests. 8th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2006.

OTHER

“Electrolytes.” Lab Tests Online. April, 11 2005. http://www.labtestsonline.org/understanding/analytes/electrolytes/test.html [Accessed April 11, 2008].

“Electrolytes.” MedLine Plus. August 14, 2007. http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm [Accessed April 11, 2008].

Stoppler, Melissa Conrad, MD, and William C. Shiel, Jr, MD, FACP, FACR. “Electrolytes.” MedicineNet. January 1, 2006. http://www.medicinenet.com/electrolytes/article.htm [Accessed April 11, 2008].

Erika J. Norris

Mark A. Best, MD

Laura Jean Cataldo, RN, EdD

Electrolyte tests

Definition

Electrolyte tests are laboratory tests usually performed on a blood sample, that measure the levels of electrolytes present in the body.

Purpose

Electrolyte tests, sometimes called an electrolyte panel, measure the levels of electrolytes in the blood. Most electrolyte tests include sodium , potassium, chloride, and bicarbonate. In many cases electrolyte tests are run as part of a general health screening. In other cases, electrolyte tests may be done to help diagnose specific diseases and conditions such as kidney failure. In the emergency room electrolyte tests are often done right away if the doctor is not sure what is causing the patient's symptoms.

Precautions

No special precautions are necessary for electrolyte tests.

Description

Electrolytes are minerals required by the body for good health. They are found in cells, in the fluid between and around cells, in the blood and other body fluids. They help the body maintain correct fluid balance, and are also necessary for the function of the brain, muscles, and a variety of organs.

QUESTIONS TO ASK YOUR DOCTOR

• When can I expect my test results?
• What is the next step if the test results are normal?
• What is the next step if the test results are abnormal?

Electrolyte levels are usually tested using a blood test. Blood is drawn from the arm or the back of the hand and sent to a laboratory for analysis. In the lab, technicians use a variety of specific tests to determine the levels of specific electrolytes in the blood. Electrolyte tests usually test the levels of:

• Chloride—Required by the body to maintain a proper balance between the fluids outside of cells and the fluids within the cell walls. It also is regulates correct blood acidity levels, blood pressure, and blood volume.
• Potassium—Plays an important role in maintaining fluid balance in the body. Important for nerve and muscle function, and helps regulate heartbeat.
• Sodium—Required for regulation of the amount of water present in the body. Necessary for neuron, brain, and muscle functioning.
• Bicarbonate—Acts to balance the acidity of the body's fluids.

Preparation

No special preparation is required for electrolyte tests.

Aftercare

No aftercare is required for electrolyte tests.

Complications

No complications are generally experienced from electrolyte tests. In some cases, bruising , excess bleeding, or infection can occur due to a blood draw.

Results

The results from the electrolyte tests are interpreted by the doctor with reference to any known diseases, conditions, or symptoms of the patient. Electrolyte levels that are increased or decreased can often be a sign of many different health problems. A few examples are:

• Chloride—Levels of chloride above normal may indicate kidney disease, excessive salt consumption, anemia, dehydration, or hyperthyroidism. Chloride levels below normal may be caused by kidney or heart failure, severe burns, complications of diabetes, or vomiting.
• Potassium—Raised potassium levels can indicate kidney damage, sever burns, kidney disease, excessive consumption of supplements containing potassium, diabetes, infection, or Addison's disease. Lowered potassium levels can indicate cystic fibrosis, excess production by the adrenal glands, diarrhea, dehydration, or vomiting.
• Sodium—High sodium levels can indicate dehydration, diabetes insipidus, increased levels of aldosterone. Low sodium levels can indicate burns, vomiting, diarrhea, heart failure, kidney disease, or excessive sweating.
• Bicarbonate—Increased bicarbonate levels can indicate pneumonia, emphysema, alcoholism, or chronic obstructive pulmonary disease. Lowered bicarbonate levels can indicate dehydration, diabetes, kidney failure, liver failure, or hyperventilation.

KEY TERMS

Hyperthyroidism —The presence of excessive amounts thyroid hormone, produced by the thyroid gland.

Caregiver concerns

A doctor determines when electrolyte tests are recommended. This may be done by a physician during a regular checkup, a doctor seen for a particular set of symptoms, or an emergency room doctor. A phlebotomist, or a nurse trained in drawing blood, draws the blood sample to used in the tests. The nurse then labels the sample and sends it to a laboratory where a laboratory technician performs the electrolyte tests. The results are then sent back to doctor who ordered to test who determines what, if any, further action is indicated.

Resources

BOOKS

Just the Facts: Fluids & Electrolytes. Philadelphia, PA: Lippincott Williams & Wilkins, 2005.

Chernecky, Cynthia C., Denise Macklin, Kathleen Murphy-Ende. Fluids & Electrolytes, 2nd Ed. St. Louis: Elsevier Saunders, 2006.

Hawkins, W. Rex. Eat Right—Electrolyte: a Nutritional Guide to Minerals in Our Daily Diet. Amherst, NY: Prometheus Books, 2006.

PERIODICALS

“Passing a Blood Test.” Science 299.5607 (Jan 31, 2003):627–629.

Sawka, Michael N., Louise M. Burke, E. Randy Eichner, Ronald J. Maughan, Scott J. Montain and Nina S. Stachenfeld. “Exercise and Fluid Replacement.” Medicine and Science in Sports and Exercise 39.2 (Feb 2007): p. 377–390.

Robert Bockstiegel