Diagnosing Diabetes and Prediabetes

A number of tests can be used to diagnose diabetes and prediabetes. They are as follows:

The HbA1c test (also known as the A1C or glycosylated hemoglobin test) measures the percentage of hemoglobin in the blood that has glucose bound to it. It can be done at any time of day, with no fasting required beforehand.

• An HbA1c level of 5.7% to 6.4% indicates a high risk of developing diabetes.

• An HbA1c level of 6.5% or higher indicates diabetes.

In the fasting plasma glucose test, a person’s plasma glucose level is measured after a fast of 8–12 hours.

• A person without diabetes or prediabetes has a plasma glucose level below 100 mg/dl.

• A person with impaired fasting glucose, or prediabetes, has a plasma glucose level between 100 and 125 mg/dl.

• If the fasting plasma glucose level rises to 126 mg/dl or above, the person has diabetes. (This must be confirmed with a second fasting plasma glucose test.)

The oral glucose tolerance test measures a person’s blood glucose level twice: once after the person has fasted for 8–12 hours, then again two hours after drinking a solution that contains glucose. This is done to see how the body responds to the glucose.

• A blood glucose level no higher than 140 mg/dl two hours after drinking the solution indicates normal glucose tolerance.

• A blood glucose level between 140 and 199 mg/dl after two hours indicates impaired glucose tolerance.

• If the two-hour blood glucose level rises to 200 mg/dl or above, the person has diabetes.

Adapted from the American Diabetes Association position statement “Diagnosis and Classification of Diabetes Mellitus.” Diabetes Care 2012;35:S64—S71.

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