How is diabetes mellitus diagnosed?

Experts have reached international agreement on the blood sugar thresholds for diagnosing diabetes. Because blood sugar levels tend to rise steeply after a meal, for example, the thresholds also depend on how long ago you last ate.

Fasting blood glucose test

This test is often done in the morning before breakfast. Diabetes is diagnosed if the fasting blood glucose level is over 126 mg/dL (7 mmol/L) in several measurements.

Glucose tolerance test

The oral glucose tolerance test (OGTT) is used to measure how well the body can process a larger amount of sugar. First, you drink a glass containing water with 75 grams of dissolved sugar. This makes blood sugar levels rise, but they should usually go back down rather quickly. Diabetes is diagnosed if blood sugar levels are still above 200 mg/dL (11.1 mmol/L) after two hours.

Random glucose test

Blood sugar levels are also often tested when blood is drawn for another reason. Levels above 200 mg/dL (11.1 mmol/L) in these random tests is a sign that you may have diabetes. If that is the case, your doctor will usually recommend a fasting blood sugar test or a glucose tolerance test.


HbA1c is a part of the red pigment in your blood called hemoglobin that sugar molecules can bind to. This measurement shows the average blood sugar level over the last three months. Diabetes is diagnosed if HbA1c is over 6.5%.