Red Blood Cell Indices

views updated May 29 2018

Red Blood Cell Indices

Definition

Red blood cell indices are measurements that describe the size and oxygen-carrying protein (hemoglobin) content of red blood cells. The indices are used to help in the differential diagnosis of anemia. They are also called red cell absolute values or erythrocyte indices.

Purpose

Anemia includes a variety of conditions with the same outcome: a person's blood cannot carry as much oxygen as it should. A healthy person has an adequate number of correctly sized red blood cells that contain enough hemoglobin to carry sufficient oxygen to all the body's tissues. An anemic person has red blood cells that are either too small or too few in number. As a result, the heart and lungs must work harder to make up for the lack of oxygen delivered to the tissues by the blood.

Anemia is caused by many different diseases or disorders. The first step in finding the cause is to determine what type of anemia the person has. Red blood cell indices help to classify the anemias.

Precautions

Certain prescription medications may affect the test results. These drugs include zidovudine (Retrovir), phenytoin (Dilantin), and azathioprine (Imuran).

Description

Overview

Anemia has several general causes: blood loss; a drop in production of red blood cells; or a rise in the number of red blood cells destroyed. Blood loss can result from severe hemorrhage or a chronic slow bleed, such as the result of an accident or an ulcer. Lack of iron, vitamin B₁₂, or folic acid in the diet, as well as certain chronic diseases, lower the number of red blood cells produced by the bone marrow. Inherited disorders affecting hemoglobin, severe reactions to blood transfusions, prescription medications, or poisons can cause red blood cells to burst (hemolyze) well before the end of their usual 120-day lifespan.

Anemia of any type affects the results of one or more of the common blood tests. These tests are the hematocrit, hemoglobin, and red blood cell count. The hematocrit is a measure of red blood cell mass, or how much space in the blood is occupied by red blood cells. The hemoglobin test is a measure of how much hemoglobin protein is in the blood. The red blood cell count (RBC) measures the number of red blood cells present in the blood. Red blood cell indices are additional measurements of red blood cells based on the relationship of these three test results.

The relationships between the hematocrit, the hemoglobin level, and the RBC are converted to red blood cell indices through mathematical formulas. These formulas were worked out and first applied to the classification of anemias by Maxwell Wintrobe in 1934.

The indices include these measurements: mean corpuscular volume (MCV); mean corpuscular hemoglobin (MCH); mean corpuscular hemoglobin concentration (MCHC); and red cell distribution width (RDW). They are usually calculated by an automated instrument as part of a complete blood count (CBC). Indices are covered by insurance when medically necessary. Results are available the same day that the blood is drawn or the following day.

Mean corpuscular volume (MCV)

MCV is the index most often used. It measures the average volume of a red blood cell by dividing the hematocrit by the RBC. The MCV categorizes red blood cells by size. Cells of normal size are called normocytic, smaller cells are microcytic, and larger cells are macrocytic. These size categories are used to classify anemias. Normocytic anemias have normal-sized cells and a normal MCV; microcytic anemias have small cells and a decreased MCV; and macrocytic anemias have large cells and an increased MCV. Under a microscope, stained red blood cells with a high MCV appear larger than cells with a normal or low MCV.

Mean corpuscular hemoglobin concentration (MCHC)

The MCHC measures the average concentration of hemoglobin in a red blood cell. This index is calculated by dividing the hemoglobin by the hematocrit. The MCHC categorizes red blood cells according to their concentration of hemoglobin. Cells with a normal concentration of hemoglobin are called normochromic; cells with a lower than normal concentration are called hypochromic. Because there is a physical limit to the amount of hemoglobin that can fit in a cell, there is no hyperchromic category.

Just as MCV relates to the size of the cells, MCHC relates to the color of the cells. Hemoglobin contains iron, which gives blood its characteristic red color. When examined under a microscope, normal red blood cells that contain a normal amount of hemoglobin stain pinkish red with a paler area in the center. These normochromic cells have a normal MCHC. Cells with too little hemoglobin are lighter in color with a larger pale area in the center. These hypochromic cells have a low MCHC. Anemias are categorized as hypochromic or normochromic according to the MCHC index.

Mean corpuscular hemoglobin (MCH)

The average weight of hemoglobin in a red blood cell is measured by the MCH. The formula for this index is the sum of the hemoglobin multiplied by 10 and divided by the RBC. MCH values usually rise or fall as the MCV is increased or decreased.

Red cell distribution width (RDW)

The RDW measures the variation in size of the red blood cells. Usually red blood cells are a standard size. Certain disorders, however, cause a significant variation in cell size.

Obtaining the blood sample

The RBC indices test requires 0.17-24 oz (5-7 ml) of blood. A healthcare worker ties a tourniquet on the person's upper arm, locates a vein in the inner elbow region, and inserts a needle into that vein. Vacuum action draws the blood through the needle into an attached tube. Collection of the sample takes only a few minutes.

Preparation

The doctor should check to see if the patient is taking any medications that may affect test results. The patient does not need to fast before the test.

Aftercare

Aftercare consists of routine care of the area around the puncture mark. Pressure is applied for a few seconds and the wound is covered with a bandage.

Risks

The primary risk is mild dizziness and the possibility of a bruise or swelling in the area where the blood was drawn. The patient can apply moist warm compresses.

Normal results

Normal results for red blood cell indices are as follows:

MCV 82-98 fl (femtoliters)
MCHC 31-37 g/dl
MCH 26-34 pg (picograms)
RDW 11.5-14.5%.

Abnormal results

The category into which a person's anemia is placed based on the indices provides a significant clue as to the cause of the anemia, but further testing is needed to confirm a specific diagnosis.

The most common causes of macrocytic anemia (high MCV) are vitamin B₁₂ and folic acid deficiencies. Lack of iron in the diet, thalassemia (a type of hereditary anemia), and chronic illness are the most common causes of microcytic anemia (low MCV). Normocytic anemia (normal MCV) can be caused by kidney and liver disease, bone marrow disorders, or excessive bleeding or hemolysis of the red blood cells.

Lack of iron in the diet and thalassemia are the most common causes of hypochromic anemia (low MCHC). Normocytic anemias are usually also normochromic and share the same causes (normal MCHC).

KEY TERMS

Anemia— A variety of conditions in which a person's blood can't carry as much oxygen as it should due to a decreased number or size of red blood cells.

Hypochromic— A descriptive term applied to a red blood cell with a decreased concentration of hemoglobin.

Macrocytic— A descriptive term applied to a larger than normal red blood cell.

Mean corpuscular hemoglobin (MCH)— A measurement of the average weight of hemoglobin in a red blood cell.

Mean corpuscular hemoglobin concentration (MCHC)— The measurement of the average concentration of hemoglobin in a red blood cell.

Mean corpuscular volume (MCV)— A measure of the average volume of a red blood cell.

Microcytic— A descriptive term applied to a smaller than normal red blood cell.

Normochromic— A descriptive term applied to a red blood cell with a normal concentration of hemoglobin.

Normocytic— A descriptive term applied to a red blood cell of normal size.

Red blood cell indices— Measurements that describe the size and hemoglobin content of red blood cells.

Red cell distribution width (RDW)— A measure of the variation in size of red blood cells.

The RDW is increased in anemias caused by deficiencies of iron, vitamin B₁₂, or folic acid. Abnormal hemoglobins, such as in sickle cell anemia, can change the shape of red blood cells as well as cause them to hemolyze. The abnormal shape and the cell fragments resulting from hemolysis increase the RDW. Conditions that cause more immature cells to be released into the bloodstream, such as severe blood loss, will increase the RDW. The larger size of immature cells creates a distinct size variation.

Resources

BOOKS

Pagana, Kathleen Deska, and Timothy James Pagana, editors. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.

Gale Encyclopedia of Medicine, 3rd ed. Nordenson, Nancy

Red Blood Cell Indices

views updated May 08 2018

Red blood cell indices

Definition

Red blood cell (RBC) indices are calculations derived from the complete blood count that aid in the diagnosis and classification of anemia. Measurements needed to calculate indices are the red blood cell count, hemoglobin, and hematocrit. The hematocrit is the percentage of blood by volume that is occupied by the red cells. The three RBC indices are:

Mean corpuscular volume (MCV). The average size of the red blood cells expressed in femtoliters. MCV is calculated by dividing the hematocrit (as percent) by the RBC count in millions per microliter of blood, then multiplying by 10.
Mean corpuscular hemoglobin (MCH). The average amount of hemoglobin inside an RBC expressed in picograms. The MCH is calculated by dividing the hemoglobin concentration in grams per deciliter by the RBC count in millions per microliter, then multiplying by 10.
Mean corpuscular hemoglobin concentration (MCHC). The average concentration of hemoglobin in the RBCs expressed as a percent. It is calculated by dividing the hemoglobin in grams per deciliter by the hematocrit, then multiplying by 100.

Purpose

Red blood cell indices help classify types of anemia, a decrease in the oxygen carrying capacity of the blood. Healthy people have an adequate number of correctly sized red blood cells containing enough hemoglobin to carry sufficient oxygen to all the body's tissues. Anemia is diagnosed when either the hemoglobin or hematocrit of a blood sample is too low.

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic. Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases, including iron deficiency anemia , thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of macrocytic anemia are vitamin B ₁₂ deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g. malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells are normal.

A low MCH indicates that cells have too little hemoglobin. This is caused by deficient hemoglobin production. Such cells will be pale when examined under the microscope and are termed hypochromic. Iron deficiency is the most common cause of a hypochromic anemia. The MCH is usually elevated in macrocytic anemias associated with vitamin B ₁₂ and folate deficiency.

The MCHC is the ratio of hemoglobin mass in the RBC to cell volume. Cells with too little hemoglobin are lighter in color and have a low MCHC. The MCHC is low in microcytic, hypochromic anemias such as iron deficiency, but is usually normal in macrocytic anemias. The MCHC is elevated in hereditary spherocytosis, a condition with decreased RBC survival caused by a structural protein defect in the RBC membrane.

Description

Cell indices are usually calculated from tests performed on an automated electronic cell counter. However, these counters measure the MCV, which is directly proportional to the voltage pulse produced as each cell passes through the counting aperture. Electronic cell counters calculate the MCH, MCHC, hematocrit, and an additional parameter called the red cell distribution width (RDW). The RDW is a measure of the variance in red blood cell size. It is calculated by dividing the standard deviation of RBC volume by the MCV and multiplying by 100. A large RDW indicates abnormal variation in cell size, termed anisocytosis. The RDW aids in differentiating anemias that have similar indices. For example, thalassemia minor and iron deficiency anemia are both microcytic and hypochromic anemias, and overlap in MCV and MCH. However, iron deficiency anemia has an abnormally wide RDW, but thalassemia minor does not.

Precautions

Certain prescription medications may affect the test results. These drugs include zidovudine (Retrovir), phenytoin (Dilantin), and azathioprine (Imuran). When the hematocrit is determined by centrifugation, the MCV and MCHC may differ from those derived by an electronic cell counter, especially in anemia. Plasma trapped between the RBCs tends to cause an increase in the hematocrit, giving rise to a somewhat higher MCV and lower MCHC.

RBC indices require 3–5 mL of blood collected by venipuncture. A nurse or phlebotomist usually collects the sample following standard precautions for the prevention of transmission of bloodborne pathogens.

Aftercare

Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated accordingly.

Risks

The risks are potential bruising at the puncture site, and mild dizziness . Rarely excess bleeding, or infection of the puncture site occurs.

Normal results

Normal results for red blood cell indices are as follows:

MCV: 78–102 fl (femtoliters) for ages 12–18 years, 77–95 fl for ages six to 12 years, 75–87 fl for ages two to six years, 70–86 fl for ages six months to two years, 85–123 fl for age one month
MCH: 25–35 pg (picograms) for ages 12–18 years, 25–33 pg for ages six to 12 years, 24–30 pg for ages two to six years, 23–31 pg for ages six months to two years, 28–40 pg for age one month
MCHC: 31–37 g/dL for ages two to 18 years, 30–36 g/dL for ages six months to two years, 29–37 pg for age one month
RDW: 12–15 percent

Parental concerns

The pain from the needle puncture only lasts a moment. The parent should comfort a child as needed. Older children can be prepared for the test ahead of time, and the reason why the test is being given should also be explained if the child is old enough to understand.

When to call a doctor

If the bleeding does not stop at the needle puncture site, or if hours to days later, there appears to be infection (redness and swelling), then parents should contact a doctor.

KEY TERMS

Anemia —A condition in which there is an abnormally low number of red blood cells in the bloodstream. It may be due to loss of blood, an increase in red blood cell destruction, or a decrease in red blood cell production. Major symptoms are paleness, shortness of breath, unusually fast or strong heart beats, and tiredness.

Hypochromic —A descriptive term applied to a red blood cell with a decreased concentration of hemoglobin.

Macrocytic —A descriptive term applied to a larger than normal red blood cell.

Mean corpuscular hemoglobin concentration (MCHC) —A measurement of the average concentration of hemoglobin in a red blood cell.

Mean corpuscular hemoglobin (MCH) —A measurement of the average weight of hemoglobin in a red blood cell.

Mean corpuscular volume (MCV) —A measurement of the average volume of a red blood cell.

Microcytic —A descriptive term applied to a smaller than normal red blood cell.

Normochromic —A descriptive term applied to a red blood cell with a normal concentration of hemoglobin.

Normocytic —A descriptive term applied to a red blood cell of normal size.

Red blood cell indices —Measurements that describe the size and hemoglobin content of red blood cells. The indices are used to help in the differential diagnosis of anemia. Also called red cell absolute values or erythrocyte indices.

Red cell distribution width (RDW) —A measure of the variation in size of red blood cells.

Resources

BOOKS

Chernecky, Cynthia C., and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures, 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.

Kee, Joyce LeFever. Handbook of Laboratory and Diagnostic Tests, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Kjeldsberg, Carl, et al. Practical Diagnosis of Hematologic Disorders, 3rd Ed. Chicago: ASCP Press, 2000.

Mark A. Best

Gale Encyclopedia of Children's Health: Infancy through Adolescence Best, Mark

Red Blood Cell Indices

views updated Jun 11 2018

Red Blood Cell Indices

Definition
Purpose
Description
Diagnosis/Preparation
Aftercare
Risks
Normal results

Definition

Red blood cell (RBC) indices are calculations derived from the complete blood count that aid in the diagnosis and classification of anemia.

Purpose

Red blood cell indices help classify types of ane-mia, a decrease in the oxygen carrying capacity of the blood. Healthy people have an adequate number of correctly sized red blood cells containing enough hemoglobin to carry sufficient oxygen to all the body’s tissues. Anemia is diagnosed when either the hemoglobin or hematocrit of a blood sample is too low.

Description

Measurements needed to calculate RBC indices are the red blood cell count, hemoglobin, and hematocrit. The hematocrit is the percentage of blood by volume that is occupied by the red cells. The three main RBC indices are:

Mean corpuscular volume (MCV). The average size of the red blood cells expressed in femtoliters (fl). MCV is calculated by dividing the hematocrit (as percent) by the RBC count in millions per microliter of blood, then multiplying by 10.
Mean corpuscular hemoglobin (MCH). The average amount of hemoglobin inside an RBC expressed in picograms (pg). The MCH is calculated by dividing the hemoglobin concentration in grams per deciliter by the RBC count in millions per microliter, then multiplying by 10.
Mean corpuscular hemoglobin concentration (MCHC). The average concentration of hemoglobin in the RBCs expressed as a percent. It is calculated by dividing the hemoglobin in grams per deciliter by the hematocrit, then multiplying by 100.

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic.

Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases, including iron deficiency anemia, thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of macrocytic anemia are vitamin B12 deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g. malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells are normal.

KEY TERMS

Anemia— A variety of conditions in which a person’s blood cannot carry as much oxygen as is needed by the tissues.

Hypochromic— A descriptive term applied to a red blood cell with a decreased concentration of hemoglobin.

Macrocytic— A descriptive term applied to a larger than normal red blood cell.

Mean corpuscular hemoglobin (MCH)— A calculation of the average weight of hemoglobin in a red blood cell.

Mean corpuscular hemoglobin concentration (MCHC)— A calculation of the average concentration of hemoglobin in a red blood cell.

Mean corpuscular volume (MCV)— A measure of the average volume of a red blood cell.

Microcytic— A descriptive term applied to a smaller than normal red blood cell.

Normochromic— A descriptive term applied to a red blood cell with a normal concentration of hemoglobin.

Normocytic— A descriptive term applied to a red blood cell of normal size.

Red cell distribution width (RDW)— A measure of the variation in the size of red blood cells.

A low MCH indicates that cells have too little hemoglobin. This is caused by deficient hemoglobin production. Such cells will be pale when examined under the microscope and are termed hypochromic. Iron deficiency is the most common cause of a hypo-chromic anemia. The MCH is usually elevated in macrocytic anemias associated with vitamin B₁₂ and folate deficiency.

The MCHC is the ratio of hemoglobin mass in the RBC to cell volume. Cells with too little hemoglobin are lighter in color and have a low MCHC. The MCHC is low in microcytic, hypochromic anemias such as iron deficiency, but is usually normal in macrocytic anemias. The MCHC is elevated in hereditary spherocytosis, a condition with decreased RBC survival caused by a structural protein defect in the RBC membrane.

Cell indices are usually calculated from tests performed on an automated electronic cell counter. However, these counters measure the MCV, which is directly proportional to the voltage pulse produced as each cell passes through the counting aperture. Electronic cell counters calculate the MCH, MCHC, hematocrit, and an additional parameter called the red cell distribution width (RDW).

The RDW is a measure of the variance in red blood cell size. It is calculated by dividing the standard deviation (a measure of variation) of RBC volume by the MCV and multiplying by 100. A large RDW indicates abnormal variation in cell size, termed anisocy-tosis. The RDW aids in differentiating anemias that have similar indices. For example, thalassemia minor and iron deficiency anemia are both microcytic and hypochromic anemias, and overlap in MCV and MCH. However, iron deficiency anemia has an abnormally wide RDW, but thalassemia minor does not.

Diagnosis/Preparation

RBC indices require 3-5 mL of blood collected by vein puncture with a needle. A nurse or phlebotomist usually collects the sample.

Aftercare

Discomfort or bruising may occur at the punctusite. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be allowed to lie down and relax until they are stable.

Risks

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test. However, certain prescription medications may affect the test results. These drugs include zidovudine (Retrovir), phenytoin (Dilantin), and azathioprine (Imuran). When the hematocrit is determined by centrifugation, the MCV and MCHC may differ from those derived by an electronic cell counter, especially in anemia. Plasma trapped between the RBCs tends to cause an increase in the hematocrit, giving rise to a somewhat higher MCV and lower MCHC.

Normal results

Normal results for red blood cell indices are as follows:

MCV: 80-96 fl
MCH: 27-33 pg
MCHC: 33-36%
RDW: 12-15%

Resources

BOOKS

Hoffman, R. et al. Hematology: Basic Principles and Practice. 4th ed. Philadelphia: Elsevier, 2005.

McPherson, R. A. et al. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia: Saunders, 2007.

OTHER

National Institutes of Health [cited April 5, 2003]. http://www.nlm.nih.gov/medlineplus/encyclopedia.html.

Victoria E. DeMoranville

Robert Harr

Mark A. Best

Rosalyn Carson-DeWitt, MD

Red blood cell test seeHemoglobin test

Regional anesthetic seeAnesthesia, local

Remote surgery seeTelesurgery

Renal transplant seeKidney transplantation

The Gale Encyclopedia of Surgery and Medical Tests

Red Blood Cell Indices

views updated Jun 11 2018

Red Blood Cell Indices

Definition

Red blood cell (RBC) indices are calculations derived from the complete blood count that aid in the diagnosis and classification of anemia. Measurements needed to calculate indices are the red blood cell count, hemoglobin, and hematocrit. The hematocrit is the percentage of blood by volume that is occupied by the red cells. The three RBC indices are:

Mean corpuscular volume (MCV). The average size of the red blood cells expressed in femtoliters. MCV is calculated by dividing the hematocrit (as percent) by the RBC count in millions per microliter of blood, then multiplying by 10.
Mean corpuscular hemoglobin (MCH). The average amount of hemoglobin inside an RBC expressed in picograms. The MCH is calculated by dividing the hemoglobin concentration in grams per deciliter by the RBC count in millions per microliter, then multiplying by 10.
Mean corpuscular hemoglobin concentration (MCHC). The average concentration of hemoglobin in the RBCs expressed as a percent. It is calculated by dividing the hemoglobin in grams per deciliter by the hematocrit, then multiplying by 100.

Purpose

Red blood cell indices help classify types of anemia, a decrease in the oxygen carrying capacity of the blood. Healthy people have an adequate number of correctly sized red blood cells containing enough hemoglobin to carry sufficient oxygen to all the body's tissues. Anemia is diagnosed when either the hemoglobin or hematocrit of a blood sample is too low.

The mechanisms by which anemia occurs will alter the RBC indices in a predictable manner. Therefore, the RBC indices permit the physician to narrow down the possible causes of an anemia. The MCV is an index of the size of the RBCs. When the MCV is below normal, the RBCs will be smaller than normal and are described as microcytic. When the MCV is elevated, the RBCs will be larger than normal and are termed macrocytic. RBCs of normal size are termed normocytic. Failure to produce hemoglobin results in smaller than normal cells. This occurs in many diseases, including iron deficiency anemia, thalassemia (an inherited disease in which globin chain production is deficient), and anemias associated with chronic infection or disease. Macrocytic cells occur when division of RBC precursor cells in the bone marrow is impaired. The most common causes of macrocytic anemia are vitamin B₁₂ deficiency, folate deficiency, and liver disease. Normocytic anemia may be caused by decreased production (e.g. malignancy and other causes of bone marrow failure), increased destruction (hemolytic anemia), or blood loss. The RBC count is low, but the size and amount of hemoglobin in the cells are normal.

A low MCH indicates that cells have too little hemoglobin. This is caused by deficient hemoglobin production. Such cells will be pale when examined under the microscope and are termed hypochromic. Iron deficiency is the most common cause of a hypochromic anemia. The MCH is usually elevated in macrocytic anemias associated with vitamin B₁₂ and folate deficiency.

The MCHC is the ratio of hemoglobin mass in the RBC to cell volume. Cells with too little hemoglobin are lighter in color and have a low MCHC. The MCHC is low in microcytic, hypochromic anemias such as iron deficiency, but is usually normal in macrocytic anemias. The MCHC is elevated in hereditary spherocytosis, a condition with decreased RBC survival caused by a structural protein defect in the RBC membrane.

Cell indices are usually calculated from tests performed on an automated electronic cell counter. However, these counters measure the MCV, which is directly proportional to the voltage pulse produced as each cell passes through the counting aperture. Electronic cell counters calculate the MCH, MCHC, hematocrit, and an additional parameter called the red cell distribution width (RDW). The RDW is a measure of the variance in red blood cell size. It is calculated by dividing the standard deviation of RBC volume by the MCV and multiplying by 100. A large RDW indicates abnormal variation in cell size, termed anisocytosis. The RDW aids in differentiating anemias that have similar indices. For example, thalassemia minor and iron deficiency anemia are both microcytic and hypochromic anemias, and overlap in MCV and MCH. However, iron deficiency anemia has an abnormally wide RDW, but thalassemia minor does not.

Precautions

Certain prescription medications may affect the test results. These drugs include zidovudine (Retrovir), phenytoin (Dilantin), and azathioprine (Imuran). When the hematocrit is determined by centrifugation, the MCV and MCHC may differ from those derived by an electronic cell counter, especially in anemia. Plasma trapped between the RBCs tends to cause an increase in the hematocrit, giving rise to a somewhat higher MCV and lower MCHC.

RBC indices require 3-5 mL of blood collected by venipuncture. A nurse or phlebotomist usually collects the sample following standard precautions for the prevention of transmission of bloodborne pathogens.

Aftercare

Discomfort or bruising may occur at the puncture site. Pressure to the puncture site until the bleeding stops reduces bruising; warm packs relieve discomfort. Some people feel dizzy or faint after blood has been drawn and should be treated accordingly.

Complications

Other than potential bruising at the puncture site, and/or dizziness, there are no complications associated with this test.

Results

Normal results for red blood cell indices are as follows:

MCV: 80-96 fl (femtoliters)
MCH: 27-33 pg (picograms)
MCHC: 33-36%
RDW: 12-15%

Health care team roles

A phlebotomist, or sometimes a nurse, collects the blood; and a clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) or clinical laboratory technician CLT(NCA)/medical laboratory technician MLT(ASCP) performs the testing. Results are interpreted by a physician.

KEY TERMS

Anemia— A variety of conditions in which a person's blood can't carry as much oxygen as is needed by the tissues.