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

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Category:

Turnaround Time: Within 1 day
CPT Code:

82947; 82950

Test Type: 1 mL serum or plasma each tube
Stability Time:

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Overview:

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.
The ADA criteria for diagnosing diabetes includes typical symptoms of diabetes (polydipsia, polyuria, and unexplained weight loss plus casual plasma glucose >200 mg/dL. "Casual" meaning any random glucose obtained at any time of the day without respect to fasting or not fasting.); plus Hb A1c ≥6.5% or fasting plasma glucose ≥126 mg/dL after no caloric intake for at least eight hours or two-hour plasma glucose ≥200 mg/dL during a 75-gram oral glucose tolerance test and any of the initial findings (above) must be confirmed on a subsequent day. OGTT is contraindicated in the presence of obvious diabetes mellitus.

1. Field JB. Hypoglycemia: A systematic approach to specific diagnosis. Hosp Pract (Off Ed). 1986 Sep 15; 21(9):187-194. PubMed 21193625

Alberti KG, Zimmet PZ. Definition, diagnosis and classification of diabetes mellitus and its complications, Part 1: Diagnosis and classification of diabetes mellitus provisional report of a WHO consultation. Diabet Med. 1998 Jul; 15(7): 539-553. PubMed 9686693

American Diabetes Association. Standards of medical care in diabetes—2015: Summary of revisions. Diabetes Care. 2015 Jan; 38(Suppl 1):S4. PubMed 25537706

Forest JC, Garrido-Russo M, LeMay A, Carrier R, Dube JL. Reference values for the oral glucose tolerance test at each trimester of pregnancy. Am J Clin Pathol. 1983 Dec; 80(6):823-831. PubMed 6356879

Hare JW. Gestational diabetes mellitus. Levels of glycemia as management goals. Diabetes. 1991 Dec; 40(Suppl 2):193-196. PubMed 1748258

Lindsay MK, Graves W, Klein L. The relationship of one abnormal glucose tolerance test value and pregnancy complications. Obstet Gynecol. 1989 Jan; 73(1):103-106. PubMed 2909030

Neiger R, Coustan DR. The role of repeat glucose tolerance tests in the diagnosis of gestational diabetes. Am J Obstet Gynecol. 1991 Oct; 165(4 Pt1):787-790. PubMed 1951534

Phelps G, Chapman I, Hall P, Braund W, Mackinnon M. Prevalence of genetic hemochromatosis among diabetic patients. Lancet. 1989 Jul 29; 2(8657):233-234. PubMed 2569052

Singer DE, Coley CM, Samet JH, Nathan DM. Tests of glycemia in diabetes mellitus. Their use in establishing diagnosis and treatment. Ann Intern Med. 1989 Jan 15; 110(2):125-137. PubMed 2642375

Collection Details:

Patient Preparation:

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.

Collection Instructions:

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

Draw a fasting blood sample before administering glucose. Administer a 75-gram glucose and draw blood after two hours. The patient should remain seated throughout the test. Submit 1 mL serum or plasma for fasting and two-hour specimens. Separate serum or plasma from cells within 45 minutes of venipuncture. Gray-top tubes only, may be submitted without centrifugation. Label each tube with the patient's name and collection time interval. (ie, fasting and two-hour).

Maintain specimen at room temperature.