The oral glucose tolerance test (OGTT) is used for individuals who are able to eat and who are not known to have problems with gastrointestinal malabsorption. The client should be in a normal nutritional state and should be capable of normal physical activity (i.e., not immobilized or on bed rest), because carbohydrate depletion and inactivity may impair glucose tolerance. In addition, drugs that affect blood glucose levels (see Table 5-2) should not be taken for several days before the test. Because oral glucose tolerance testing is affected by so many variables, the results are subject to many diagnostic interpretations.¹⁵

The OGTT may be performed using blood samples only or using urine samples as well. The urine is normally negative for sugar throughout the test; that is, because the average renal threshold for glucose is 180 mg/dL, the plasma glucose level must be approximately 180 mg/dL before sugar appears in the urine. Renal threshold levels vary, however, and urine testing during an OGTT may show how much glucose the individual spills, if any, at various blood glucose levels. As long as the renal threshold is not surpassed by the blood glucose levels, all of the glucose presented to the kidneys is reabsorbed from the glomerular filtrate by the renal tubules, provided that renal function is normal.

[Show Table Outline]

Time After Carbohydrate Challenge 30 min

	Conventional Units	SI Units
Whole blood glucose	<150 mg/dL	8.3 mmol/L
Serum/plasma	<160 mg/dL	>8.8 mmol/L
Urine glucose	Negative throughout test

Time After Carbohydrate Challenge 1 Hr

	Conventional Units	SI Units
Whole blood glucose	<160 mg/dL	8.8 mmol/L
Serum/plasma	<170 mg/dL	>9.4 mmol/L
Urine glucose	Negative throughout test

Time After Carbohydrate Challenge 2 Hr

	Conventional Units	SI Units
Whole blood glucose	<115 mg/dL	6.6 mmol/L
Serum/plasma	<125 mg/dL	>7.1 mmol/L
Urine glucose	Negative throughout test

Time After Carbohydrate Challenge 3 Hr

Note: Values for children over age 6 years are the same as those for adults. Values for elderly individuals are 10 to 30 mg/dL higher at each interval because of the age-related decline in glucose clearance.

[Table Outline]

• Time After Carbohydrate Challenge 30 min
• Time After Carbohydrate Challenge 1 Hr
• Time After Carbohydrate Challenge 2 Hr
• Time After Carbohydrate Challenge 3 Hr

• Failure to ingest a diet with sufficient carbohydrate content (e.g., 150 g/day) for at least 3 days before the test may result in falsely decreased values.
• Impaired physical activity may lead to falsely increased values.
• Excessive physical activity before or during the test may lead to falsely decreased values.
• Smoking before or during the test may lead to falsely increased values.
• Ingestion of drugs known to alter blood glucose levels may lead to falsely increased or decreased values (see Table 5-2).

• Abnormal fasting or postprandial blood glucose levels that are not clearly indicative of diabetes mellitus
• Identification of impaired glucose metabolism without overt diabetes mellitus, which is characterized by a modest elevation in blood glucose after 2 hours and a normal level after 3 hours
• Evaluation of glucose metabolism in women of childbearing age, especially those who are pregnant and have a history of previous fetal loss, birth of babies weighing 9 pounds or more, or positive family history for diabetes mellitus
• Support for diagnosing hyperthyroidism and alcoholic liver disease, which are characterized by a sharp rise in blood glucose followed by a decline to subnormal levels
• Identification of true postprandial hypoglycemia (5-hour GTT) caused by excessive insulin response to a glucose load
• Support for diagnosing gastrointestinal malabsorption, which is characterized by peak glucose levels lower than that normally expected and hypoglycemia in the latter hours of the test (5-hour GTT)
• Identification of abnormal renal tubular function, if glycosuria occurs without hyperglycemia

Nursing Care Before the Procedure

Explain to the client:

• The general procedure for the test, including the administration of glucose and the frequency of collection of blood and urine samples
• The importance of eating a diet containing at least 150 g carbohydrate per day for 3 days before the test (Provide sample menus or lists of foods that demonstrate how this may be accomplished.)
• Which medications, if any, are to be withheld before the test
• That no food may be eaten after midnight before the test but that water is not restricted
• The importance of not smoking or performing strenuous exercise after midnight before the test and until the test is completed
• The symptoms of hypoglycemia and the necessity of reporting such symptoms immediately

Nursing Alert

• Individuals with fasting blood sugars of greater than 150 mg/dL or postprandial blood glucose levels greater than 200 mg/dL should not receive the glucose load required for this test.
• If the client vomits the oral glucose preparation, notify the laboratory and physician immediately, and implement any treatment ordered.
• If signs and symptoms of hypoglycemia are observed or reported, obtain a blood sugar immediately and administer orange juice with 1 tsp of sugar or other beverage containing sugar; notify the physician that the test has been terminated.

Provide containers for collection of urine samples.

A venipuncture is performed and a sample is obtained for a fasting blood sugar. At the same time, a second voided (double-voided) urine sample is collected and tested for glucose. To collect a second-voided specimen, have the client void 30 minutes before the required specimen is due. Discard this urine, then collect the second voided specimen at the designated time.

The glucose load is administered orally. This is a calculated dose, either 1.75 g/kg body weight or 50 g/m² body surface. Several commercial preparations are available that are flavored for palatability. Blood and urine samples are obtained at ½-hour, 1-hour, 2-hour, and 3-hour intervals. The second voided urine specimen is necessary only at the beginning of the test. The client should drink one glass of water each time a urine sample is collected to ensure adequate urinary output for remaining specimens. If the test is extended to 5 hours, additional samples are collected at 4- and 5-hour intervals.

The test may be performed with blood samples only, depending on the desired information to be obtained from the test.

Nursing Care After the Procedure

Care and assessment after the procedure are essentially the same as those for any test involving the collection of peripheral blood samples.

• Resume food and medications withheld before the test, as well as usual activities.
• Complications and precautions: Same as for blood glucose. Closely monitor those clients whose pretest levels are greater than 200 mg/dL for possible reactions to the additional glucose intake required for the test.