June 27, 2015

Oral Glucose Tolerance Test Interpretation


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 to 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?

Please choose one:
a). Normal glucose tolerance
b). Impaired fasting gluciose
c). Impaired glucose tolerance
d). Diabetes mellitus type 1
e). Diabetes mellitus type 2


The correct answer is C


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.

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