Q&A: Oral Glucose Tolerance Test Interpretation

Tests that can be used to screen for type 2 diabetes are measurement of fasting plasma glucose, a glycated hemoglobin (A1C or HbA1C), and a two-hour plasma glucose during an oral glucose tolerance test (OGTT). However, because of its inconvenience, OGTT is not commonly used for screening, except in pregnant women.

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In this article:
Oral Glucose tolerance test
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

Glucose tolerance test

The glucose tolerance test involves oral glucose given to a subject and blood samples taken afterward to determine how quickly it is cleared from the blood. The test is usually to test for diabetes, insulin resistance, impaired beta cell function, and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. In the most commonly performed version of the test, an oral glucose tolerance test (OGTT), a standard dose of glucose is ingested by mouth and blood levels are checked two hours later.

The OGTT can be performed as a 75 gram two-hour test or a 100 gram three-hour test; there is no consensus regarding the optimum thresholds for a positive test. Although the 100 gram three-hour GTT is typically performed as the second step of the two-step approach while the 75 gram two-hour test is performed as the only test in the one-step approach, this is arbitrary. In fact, the Canadian Diabetes Association clinical guidelines suggest the 75 gram two-hour GTT as the second step of the two-step approach. Historically, carbohydrate loading for three days before the test was recommended but is often no longer recommended and is probably not necessary if the patient is not on a low-carbohydrate diet.

MCQ exam: clinical scenario

A 45-year-old woman presented for a full medical check up. She was overweight (body mass index [BMI], 29 kg/m2) but appeared to have no other obvious health problems. The woman was very concerned that she could have diabetes because she had read on the internet that sometimes people with insulin resistance have trouble with their weight. Therefore, her doctor organised an oral glucose tolerance test (OGTT) with a 75 g glucose load, and simultaneous insulin levels. Her normal fasting blood sugar level of 120 mg/dl rose up to 150 mg/dl after the first 60 minutes and this was associated with her feeling quite unwell and tired. By the end of the second hour, the blood glucose level had risen to 180 mg/dl.

How would you classify this result?

a). Normal glucose tolerance
b). Impaired fasting glucose
c). Impaired glucose tolerance
d). Diabetes mellitus type 1
e). Diabetes mellitus type 2

MCQ exam: answer

The correct answer is C.

MCQ exam: explanation

People with impaired glucose tolerance (IGT) have blood glucose levels that are higher than normal but not high enough to say they have diabetes. This condition is diagnosed using the oral glucose tolerance test (OGTT). After a fast of 8 to 12 hours, a person's blood glucose is measured before and 2 hours after drinking a glucose-containing solution. The possible result scenarios:

• In normal glucose tolerance, blood glucose rises no higher than 140 mg/dl 2 hours after the drink.
• In impaired glucose tolerance (IGT), the 2-hour blood glucose is between 140 and 199 mg/dl.
• If the 2-hour blood glucose rises to 200 mg/dl or above, a person has diabetes.

The OGTT includes measures of blood glucose levels after a fast and after a glucose challenge.
In 1997, an American Diabetes Association (ADA) expert panel recommended that doctors use the fasting blood glucose test to screen their patients for diabetes because the test is easier and less costly than the OGTT. Though the fasting glucose test detects most diabetes cases, the OGTT is more sensitive in identifying people with blood glucose problems that may first appear only after a glucose challenge.

Reference(s)
1). DeFronzo RA, Abdul-Ghani M (2011). "Assessment and treatment of cardiovascular risk in prediabetes: impaired glucose tolerance and impaired fasting glucose". American Journal of Cardiology. 108 (3 Suppl): 3B–24B. doi:10.1016/j.amjcard.2011.03.013. PMID 21802577.
2). Institute for Quality and Efficiency in Health Care. "Glucose tolerance test: how does it work exactly?". Informed Health Online. Institute for Quality and Efficiency in Health Care.