Reviewed by Bruce J. Lanser, MD

Diabetes: Diagnosis

The fasting blood glucose test is the preferred test for diagnosing diabetes in children and non-pregnant  adults. It is most reliable when done in the morning. However, a diagnosis of diabetes can be made based on any of the following test results:
  • Fasting blood glucose test. A blood glucose level of 126 milligrams per deciliter (mg/dL) or more after an 8-hour fast.
  • Oral glucose tolerance test. A blood glucose level of 200 mg/dL or more,  2 hours after drinking a beverage containing 75 grams of glucose dissolved in water
  • A random (taken at any time of day) blood glucose level of 200 mg/dL or more, along with the presence of diabetes symptoms, on 2 or more occasions.
  • Hemoglobin A1C. An A1C of equal to or greater than 6.5%.

Gestational diabetes is diagnosed based on blood glucose levels measured during the oral glucose tolerance test, though the cutoffs for diagnosis are lower because glucose levels are normally lower during pregnancy.

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