Glucose Tolerance Test (GTT), Two-hour (Oral WHO Protocol)

Serum or plasma

1 mL serum or plasma each tube

0.5 mL serum or plasma each tube

Gel-barrier tubes (2) or gray-top (sodium fluoride/potassium oxalate plasma) tubes (2)

Patient should be active and eat a regular diet that includes at least 150 grams of carbohydrate daily for three days prior to the test. Patient should be instructed not to eat or drink anything except water for at least eight hours and not more than 14 hours before the test. Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.

The GTT only establishes the presence of glucose intolerance. It is used in patients with borderline fasting and postprandial glucose to support or rule out the diagnosis of diabetes mellitus. Some use it in unexplained hypertriglyceridemia, neuropathy, impotence, diabetes-like renal diseases, retinopathy, reevaluation of prior diagnosis made under substandard conditions and with necrobiosis lipoidica diabeticorum.

The OGTT is used to work up glycosuria with- out hyperglycemia (eg, to work up renal glycosuria). It is used to predict perinatal morbidity in pregnancy, to diagnose gestational diabetes. Risks of fetal abnormality and perinatal mortality are increased with abnormal carbohydrate metabolism in pregnancy.

When a glucose level <50 mg/dL coincides with symptoms of hypoglycemia, a six-hour glucose tolerance test is advocated,¹ but many consider the alternative better. Glucose intolerance is due to obesity in some subjects. Abnormal curves may be caused by Cushing's syndrome, pheochromocytoma, or acromegaly.

Emesis is probably an indication to cancel the remainder of a GGT for that day; decision is up to the patient's physician. Excessive growth hormone, adrenocortical and thyroid hormones, and catecholamines cause decreased glucose tolerance. Diabetes is much more than glucose intolerance, but until now we have not been able to measure other factors pertinent to prediction of the complications of diabetes. The glucose tolerance test lacks specificity and sensitivity for the complications of diabetes mellitus. Some feel that it only determines glucose intolerance. Impaired glucose tolerance is a quasi-entity; 1% to 5% of such patients become overtly diabetic yearly. Such patients have increased risk for cardiovascular disease. An increased prevalence of idiopathic hemochromatosis exists in the diabetic population compared to the general population.

Few indications still meet wide acceptance. Slight hyperglycemic effect is seen in patients on oral contraceptives. Failure to have patient on three-day high carbohydrate diet may result in a false-positive GTT. Impaired glucose tolerance is not equivalent to diabetes mellitus. A normal result does not ensure that diabetes will not subsequently develop.

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