Oral Glucose Tolerance Test

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The oral glucose tolerance test (OGTT) is one of the test used to diagnose diabetes mellitus (another being fasting blood sugar). The test is done by having the patient ingest 75 grams of glucose. The blood sugar level is then determined one and two hours after ingestion.

Outcomes

According to the 2005 Recommendation of the ADA, the possible outcomes of the oral glucose tolerance test are the following:

If the two-hour value is <140 mg/dl (7.8 mmol/l), the glucose tolerance is normal.

If the two-hour value is 140199 mg/dl (7.811.1 mmol/l), the diagnosis is impaired glucose tolerance.

If the two-hour value is ≥200 mg/dl (11.1 mmol/l), a provisional diagnosis of diabetes is made.

Rationale

The rationale behind the oral glucose tolerance test is related to carbohydrate metabolism. Upon ingestion of the glucose load, the pancreas releases insulin. Insulin causes glucose to enter the cells and lowers the glucose level in the blood. Two hours after ingestion, the blood glucose level would be below 7.8 mmol/L (140 mg/dl).

In a patient with diabetes, the pancreas does not release a sufficient amount of insulin in response to the glucose load. This causes the blood glucose level to remain elevated (greater than 11.1 mmol/L) for up to two hours after ingestion of glucose.

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