Glucose Tolerance Test (GTT), Five Specimens

Overview:

Indications vary somewhat between authorities. Some use theOGTT in individuals whose FBS varies between 115-150 mg/dL on two or more occasions; others would use further fasting plasma glucose and 2-hour postprandial glucose determina- tions for such subjects. Some use the GTT infrequently or not at all. The GTT only establishes the presence of glucose intoler- ance. 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 hyper- triglyceridemia, neuropathy, impotence, diabetes-like renal diseases, retinopathy, re-evaluation of prior diagnosis madeunder 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 diag- nose gestational diabetes. Risks of fetal abnormality and perinatal mortality are increased with abnormal carbohydratemetabolism in pregnancy.[1] When a glucose level <50 mg/dL coincides with symptoms of hypoglycemia, a 6-hour glucose tolerance test is advocated, [2] but many consider the alternative better. Glucose intolerance is due to obesity in some subjects. Ab- normal curves may be caused by Cushing's syndrome, pheo- chromocytoma or acromegaly. Emesis is probably an indication to cancel the remainder of a GGT for that day; decision is up to the patient's physi- cian. Excessive growth hormone, adrenocortical and thyroid hormones and catecholamines cause decreased glucose toler- ance. 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 sensitivityfor 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 be- come overtly diabetic yearly. Such patients have increased risk for cardiovascular disease. An increased prevalence of idiopathic hemochromatosis existsin the diabetic population compared to the general population.[3]

Few indications still meet wide acceptance. Slight hyper-glycemic effect is seen in patients on oral contraceptives. Failure to have patient on 3-day high carbohydrate diet may result in a false-positive GGT. Impaired glucose tolerance is NOT equivalent to diabetes mellitus. A normal result does not ensure that diabetes will not subsequently develop.