Oral Glucose Tolerance Test

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Oral Glucose Tolerance Test

Definition
Purpose
Precautions
Description
Preparation
Aftercare
Risks
Results

Definition

The oral glucose tolerance test (OGTT) is a series of blood tests that are used to evaluate an individual’s response to drinking a standard quantity of a specific glucose-containing solution. The patient drinks the solution, and then the blood is drawn at specific intervals over the next several hours. Each blood test measures the amount of glucose (a particular form of simple sugar) in the blood. The tests are used to evaluate patients for the possibility that they have diabetes.

When carbohydrates are ingested, they are broken down in the intestines into component parts, including sugars such as glucose. Glucose is absorbed from the small intestine into the bloodstream. It circulates throughout the body and is used by all of the body’s tissues and organs to generate the energy necessary for their normal functioning. In order for glucose to enter the body’s cells, insulin must be present. Insulin is a hormone produced in and excreted by the pancreas. Insulin functions to allow the transport of glucose into the cells of the body, as well as being involved in the body’s storage of excess glucose in the form of glycogen or triglycerides.

The blood levels of glucose and insulin are intimately related. When carbohydrates are metabolized after a meal, the blood glucose begins to rise. Under normal circumstances, the pancreas then secretes insulin, in an amount relative to the blood glucose elevation. Between meals, or after heavy exertion, glucose levels may begin to drop below a safe threshold for the body’s cells (particular cells of the brain and nervous system). In response to this lowering of blood glucose, the pancreas secretes a different hormone, called glucagon. Glucagon prompts the liver to convert glycogen into glucose, thereby elevating the blood glucose back into a safe range.

Abnormal levels of blood glucose can be life-threatening. High blood glucose is termed hyperglycemia; low blood glucose is termed hypoglycemia. Either of these conditions can result in organ failure, severe brain damage, coma, or death. Diabetes occurs when the pancreas fails to produce normal amounts of insulin, or when it completely stops producing any insulin at all (this is often referred to as insulin-dependent or type I diabetes). Diabetes can also occur when cells of the body become less responsive to the effects of insulin (this is often referred to as insulin-resistance, or type II diabetes). Diabetes causes abnormal perturbations of the serum glucose level. Over time, chronic high levels of serum glucose (which may occur in poorly controlled diabetes) can result in severe damage to the heart, the eyes, the kidneys, the circulatory system, and the nervous system. In diabetics, sudden, acute increases in the serum glucose level can result in the condition called diabetic ketoacidosis, in which the extremely high levels of blood glucose lead a life-threatening illness. Diabetics can also suffer from sudden drops in serum glucose levels; if untreated, glucose deprivation affecting the organs and tissues of the body can also be life-threatening.

Women are at risk of developing gestational diabetes during pregnancy. While this condition usually resolves after the birth of the baby, and rarely leads to a permanent diagnosis of diabetes, untreated gestational diabetes can result in problems for the baby as well as the mother. Gestational diabetes in early pregnancy can cause birth defects (particularly of the brain and/or heart) and increase the chance of miscarriage. Gestational diabetes in the second and third trimesters can cause the baby to grow very large (termed macro-somia). The baby’s size can result in problems for the mother during labor and delivery. Additionally, once the baby is born, it can suffer sudden hypoglycemia. In utero, the baby will have acclimated to its mother’s high serum glucose levels by producing high levels of insulin. After birth, suddenly deprived of that glucose, the baby’s relatively high insulin levels can result in severe hypoglycemia. If the mother is known to have gestational diabetes, then the baby will be monitored more carefully for potential drops in its blood glucose, and, if necessary, treatment with an IV solution containing glucose can be instituted rapidly.

Purpose

An oral glucose tolerance test is usually performed when there is a suspicion that an individual has Type II diabetes, for example when a serum glucose level has revealed an abnormality, when there is a strong family history of diabetes, when an individual has specific risk factors for diabetes (such as being overweight), or when an individual is experiencing symptoms suggestive of diabetes (excessive thirst and/or hunger, urinary frequency, unintentional weight loss, severe fatigue and weakness, and poor healing). Additionally, an oral glucose tolerance test is almost always ordered as a routine part of prenatal care during the second trimester of pregnancy, usually between 24 and 28 weeks of pregnancy.

Precautions

Serum glucose levels can be affected by a number of medications. Patients who are on these medications should inform their doctor, so that test results can be interpreted appropriately. Medications that may affect serum glucose levels include birth control pills, high blood pressure medications, phenytoin, furosemide, triamterene, hydrochlorothiazide, niacin, pro-pranolol, and steroid medications. Additionally, the use of alcohol, caffeine, or recent illness, infection or emotional distress may affect test results.

Patients who are taking anticoagulant medications should inform their healthcare practitioner, since this may increase their chance of bleeding or bruising after a blood test.

Description

This test requires blood to be drawn from a vein (usually one in the forearm), generally by a nurse or phlebotomist (an individual who has been trained to draw serum). The initial blood draw is performed prior to the patient drinking the glucose solution. The other blood draws are performed at set intervals over the three or so hours after the solution has been ingested.

A tourniquet is applied to the arm above the area where the needle stick will be performed. The site of the needle stick is cleaned with antiseptic, and the needle is inserted. The serum is collected in vacuum tubes. After collection, the needle is withdrawn, and pressure is kept on the serum draw site to stop any bleeding and decrease bruising. A bandage is then applied.

Preparation

The oral glucose tolerance test should only be performed when the individual is in perfectly good health and normally ambulatory/active. For the 72 hours prior to undergoing the OGTT, the individual should be instructed to eat a high-carbohydrate diet (150-200 grams of carbohydrate per day). The test is done on a fasting basis, meaning that nothing should be eaten or drunk after midnight prior to the test (the

KEY TERMS

Gestational diabetes— A type of diabetes that occurs during pregnancy. Untreated, it can cause severe complications for the mother and the baby. However, it usually does not lead to long-term diabetes in either the mother or the child.

Glucose— A simple sugar that is the product of carbohydrate metabolism. It is the major source of energy for all of the organs and tissues of the body.

Glucagon— A hormone produced in the pancreas that is responsible for elevating blood glucose when it falls below a safe level for the body’s organs and tissues.

Glycogen— The form in which glucose is stored in the body.

Hyperglycemia— Elevated blood glucose levels.

Hypoglycemia— Low blood glucose levels.

Insulin— A hormone produced by the pancreas that is responsible for allowing the body’s cells to utilize glucose. The deficiency or absence of insulin is one of the causes of the disease diabetes.

Ketoacidosis— A potentially life-threatening condition in which abnormally high blood glucose levels result in the blood become too acidic.

Macrosomia— The term used to describe a newborn baby with an abnormally high birth weight.

Pancreas— An organ located near the liver and stomach, responsible for various digestive functions. The pancreas produces insulin and glucagon, hormones that are responsible for maintaining safe blood levels of glucose.

fast should last a minimum of eight and a maximum of sixteen hours). The morning of the exam, the individual should be instructed not to smoke or drink any caffeinated beverages.

Upon arrival at the laboratory, a baseline fasting serum blood glucose will be drawn. Following this, the individual will be asked to drink a solution that contains 75 grams of glucose (pregnant women will drink a 100-gram solution). The solution must be ingested in no more than five minutes. Serum blood glucose levels will be drawn at 30- to 60-minute intervals over the next several hours. A classic oral glucose tolerance test involves five blood draws over the course of the three hour testing period. An abbreviated oral glucose tolerance involves a baseline serum glucose determination, a two-hour wait, and then a second serum glucose level.

Aftercare

As with any blood tests, discomfort, bruising, and/or a very small amount of bleeding is common at the puncture site. Immediately after the needle is withdrawn, it is helpful to put pressure on the puncture site until the bleeding has stopped. This decreases the chance of significant bruising. Warm packs may relieve minor discomfort. Some individuals may feel briefly woozy after a serum test, and they should be encouraged to lie down and rest until they feel better.

Risks

Basic blood tests, such as serum glucose levels, do not carry any significant risks, other than slight bruising and the chance of brief dizziness.

Results

Normal results of a random serum glucose test range from 70-125 milligrams per deciliter (mg/dL). A normal serum glucose level at the two-hour point (two hours after ingesting the glucose solution) is less than 140 mg/dL. During the course of the two hours, any serum glucose levels should be less than 200 mg/dL.

High levels

High serum glucose levels suggest the possibility of diabetes. However, a single high, random serum glucose level is not sufficient for definitively diagnosing diabetes. The American Diabetes Association has specific criteria that must be met in order to diagnose diabetes. They require that results are verified through testing on a minimum of two different days. The oral glucose tolerance is considered to be positive for diabetes when the blood draw at the two-hour point measures 200 mg/dL or higher.

Some individuals don’t meet the criteria for an actual diagnosis of diabetes, but have a higher-than-normal fasting serum glucose level, also known as an impaired fasting glucose (ranging from 100 mg/dL to 125 mg/dL), and an elevated 2-hour serum glucose level (ranging between 140 and 199 mg/dL). These individuals are thought to have an increased risk of eventually developing diabetes, and should be followed closely. Some practitioners believe that these serum glucose levels are indicative of “prediabetes.”

When an oral glucose tolerance test is performed on a pregnant woman, gestational diabetes is diagnosed if the test reveals any two of the following criteria:

A fasting serum glucose level greater than 95 mg/dL
A serum glucose level greater than 180 mg/dL, one hour after ingesting the standardized glucose solution

A serum glucose level greater than 155 mg/dL, two hours after ingesting the standardized glucose solution
A serum glucose level greater than 140 mg/dL, three hours after ingesting the standardized glucose solution

A number of conditions other than diabetes can cause high serum glucose levels, including:

Severe stress
Heart attack
Stroke
Cushing’s syndrome
Steroid medications
Pheochromocytoma
Polycystic ovarian disease
Acromegaly (elevated growth hormone)

Low levels

Low serum glucose levels may be due to:

The presence of an insulinoma (a tumor that secretes insulin)
Addison’s disease
Hypothyroidism (underactive thyroid)
Pituitary gland tumor
Liver disease, including cirrhosis
Kidney disease
Malnutrition
Eating disorders, including anorexia nervosa
Inappropriate doses of medicines used to treat diabetes, such as insulin or oral hypoglycemic agents