In a healthy individual, the insulin response to a large oral glucose dose is almost immediate. It peaks in 30–60 minutes and returns to normal levels within 3 hours when sufficient insulin is present to metabolize the glucose ingested at the beginning of the test. The test should be performed according to WHO guidelines using glucose load containing the equivalent of 75 g of anhydrous glucose dissolved in water or other solution.
If fasting and postload glucose test results are borderline, the GTT can support or rule out a diagnosis of diabetes; it can also be a part of a workup for unexplained hypertriglyceridemia, neuropathy, impotence, kidney diseases, or retinopathy. This test may be ordered when there is sugar in the urine or when the fasting blood sugar level is significantly elevated. The GTT/OGTT should not be used as a screening test in nonpregnant adults or children.
The GTT/OGTT is done on certain patients, with the following indications (few indications still meet wide acceptance):
Family history of diabetes
Obesity
Unexplained episodes of hypoglycemia
History of recurrent infections (boils and abscesses)
In women, history of delivery of large infants, stillbirths, neonatal death, premature labor, and spontaneous abortions
Transitory glycosuria or hyperglycemia during pregnancy, surgery, trauma, stress, MI, and ACTH administration
FPG:
Adults: 100 mg/dL or 5.6 mmol/L
120-Minute (2-Hour GTT Test) 2-Hour PG after 75-g Glucose Load:
Adults: <140 mg/dL or 7.8 mmol/L
Clinical Alert
Test results of 140–199 mg/dL (7.8–11.0 mmol/L), IGT is considered prediabetes and 200 mg/dL or 11.1 mmol/L may indicate diabetes
This is a timed test for glucose tolerance. A 2-hour PG test is done after glucose load to detect diabetes in individuals other than pregnant women.
A 5-mL sample of venous blood is drawn (fasting sample). Serum or gray-topped tubes are used. After the blood is drawn, the patient drinks all of a specially formulated glucose solution within a 5-minute time frame.
Another blood sample is obtained 2 hours after glucose ingestion.
Tolerance tests can also be performed for pentose, lactose, galactose, and D-xylose.
The GTT is not indicated in these situations:
Persistent fasting hyperglycemia >140 mg/dL or >7.8 mmol/L
Persistent fasting normal PG
Patients with overt diabetes
Persistent 2-hour PG >200 mg/dL or >11.1 mmol/L
Test has limited value in diagnosis of diabetes in children and is rarely indicated for that purpose.
Procedural Alert
GTT is contraindicated in patients with a recent history of surgery, MI, or labor and delivery—these conditions can produce invalid values.
The GTT should be postponed if the patient becomes ill, even with common illnesses such as the flu or a severe cold.
Record and report any reactions during the test. Weakness, faintness, and sweating may occur between the second and third hours of the test. If this occurs, a blood sample for a glucose level should be drawn immediately and the GTT aborted.
Should the patient vomit the glucose solution, the test is declared invalid; it can be repeated in 3 days (~72 hours).
The presence of abnormal GTT values (decreased tolerance to glucose) is based on the International Classification for Diabetes Mellitus and the following glucose intolerance categories:
In the absence of unequivocal hyperglycemia, at least two GTT values must be abnormal for a diagnosis of diabetes to be validated (from the same sample or two separate test samples).
In cases of overt diabetes, no insulin is secreted; abnormally high glucose levels persist throughout the test.
Glucose values that fall above normal values but below the diagnostic criteria for diabetes or IGT should be considered nondiagnostic.
See Table 6.3 for an interpretation of glucose tolerance levels.
Decreased glucose tolerance occurs with high glucose values in the following conditions:
Diabetes
Postgastrectomy
Hyperthyroidism
Excess glucose ingestion
Hyperlipidemia types III, IV, and V
Hemochromatosis
Cushing disease (steroid effect)
Central nervous system (CNS) lesions
Pheochromocytoma
Decreased glucose tolerance with hypoglycemia can be found in persons with von Gierke disease, severe liver damage, or increased epinephrine levels.
Increased glucose tolerance with flat curve (i.e., glucose does not increase but may decrease to hypoglycemic levels) occurs in the following conditions:
Pancreatic islet cell hyperplasia or tumor
Poor intestinal absorption caused by diseases such as sprue, celiac disease, or Whipple disease
Hypoparathyroidism
Addison disease
Liver disease
Hypopituitarism, hypothyroidism
Pretest Patient Care
Explain test purpose and procedure. A written reminder may be helpful.
Have the patient follow a normal diet before the test, with fasting 8–10 hours before the test (water is permitted).
Ensure that the following drugs are discontinued 3 days before the test because they may influence test results:
Hormones, oral contraceptives, steroids
Salicylates, anti-inflammatory drugs
Diuretic agents
Hypoglycemic agents
Antihypertensive drugs
Anticonvulsants (see Appendix E)
Insulin and oral hypoglycemic agents should be withheld on the day of testing until the test is completed.
Record the patient’s weight:
Pediatric doses of glucose are based on body weight, calculated as 1.75 g/kg not to exceed a total of 75 g
Nonpregnant adults: 75 g of glucose
Collect blood specimens at the prescribed times and record exact times collected. Urine glucose testing is no longer recommended.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Have the patient resume normal diet and activities at the end of the test. Encourage eating complex carbohydrates and protein if permitted.
Administer prescribed insulin or oral hypoglycemic agents when the test is done. Arrange for the patient to eat within a short time (30 minutes) after these medications are taken.
Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient appropriately. Patients newly diagnosed with diabetes will need diet, medication, and lifestyle modification instructions.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
Clinical Alert
If fasting glucose is >140 mg/dL (>7.8 mmol/L) on two separate occasions, or if the 2-hour postprandial blood glucose is >200 mg/dL (>11.1 mmol/L) on two separate occasions, GTT is not necessary for a diagnosis of diabetes to be established.
The GTT is of limited diagnostic value for children.
Smoking increases glucose levels.
Altered diets (e.g., weight reduction) before testing can diminish carbohydrate tolerance and suggest “false diabetes.”
Glucose levels normally tend to increase with aging.
Prolonged oral contraceptive use causes significantly higher glucose levels in the second hour or in later blood specimens.
Infectious diseases, illnesses, and operative procedures affect glucose tolerance. Two weeks of recovery should be allowed before performing the test.
Certain drugs impair glucose tolerance levels (this list is not all inclusive; see Appendix E for other drugs).
If possible, these drugs should be discontinued for at least 3 days before testing. Check with the healthcare provider for specific orders:
Insulin
Oral hypoglycemic agents
Large doses of salicylates, anti-inflammatory drugs
Thiazide diuretic agents
Oral contraceptives
Corticosteroids
Estrogens
Phenothiazines
Metyrapone
Prolonged bed rest influences GTT results. If possible, the patient should be ambulatory. A GTT in a hospitalized patient has limited value.