OGTT

The oral glucose tolerance test (OGTT) is a diagnostic procedure used to evaluate how efficiently the body metabolizes glucose. It is commonly employed to diagnose diabetes mellitus, assess impaired glucose tolerance, and screen for gestational diabetes. The test involves measuring plasma glucose after an overnight fast and again after a standardized oral glucose load.

Procedure and typical protocol

After an overnight fast, a baseline blood sample is drawn to determine fasting glucose. The patient then drinks a solution containing a specified amount of glucose dissolved in water. In adults, the standard load is 75 grams; some contexts and gestational diabetes protocols may use 100 grams. Blood glucose is then measured at predefined intervals, most commonly 2 hours after the glucose load, with additional measurements at 1 hour used in certain guidelines. The procedure requires careful timing and consistent preparation, including avoidance of food or certain medications prior to testing as advised by guidelines.

Interpretation

For non-pregnant adults using a 75 g load, a fasting glucose of 126 mg/dL (7.0 mmol/L) or a 2-hour value of 200 mg/dL (11.1 mmol/L) or higher is diagnostic of diabetes. A 2-hour value in the range of 140–199 mg/dL indicates impaired glucose tolerance, a form of prediabetes. In clinical practice, results are often interpreted alongside other measures, such as HbA1c or fasting glucose, according to local guidelines.

Gestational diabetes

Many guidelines use a one-step 75 g OGTT for gestational diabetes, with criteria such as fasting ≥ 92 mg/dL, 1-hour ≥ 180 mg/dL, or 2-hour ≥ 153 mg/dL; meeting or exceeding any one threshold may be diagnostic, though approaches vary by region. Some places also employ a two-step approach with a screening test followed by a diagnostic test.

Limitations and considerations

OGTT results can be influenced by acute illness, pregnancy, medications, or recent dietary changes. The test can be time-consuming and may cause nausea due to the glucose load. Alternatives include fasting plasma glucose or HbA1c in appropriate clinical contexts.