Harvard Health Letter

By the way, doctor: Can you help me make sense of these diabetes tests?

Q. What does it mean if my fasting blood sugar level is high (135–145) but my glycosylated hemoglobin test is negative?

A. The American Diabetes Association (ADA) defines diabetes as having a fasting blood sugar level of 126 mg/dL or higher on at least two occasions. Hemoglobin is the molecule in red blood cells that serves mainly as an escort for oxygen. But it also combines with sugar, or glycosylates. Measuring the percentage of hemoglobin that's glycosylated is a good way to assess whether you've had high blood sugar over the past several months. The blood sugar test is more of a snapshot.

The ADA doesn't recommend using glycosylated hemoglobin levels for diagnosis, partly because the definition of normal varies from lab to lab. But in reality, I and many other doctors order the test, often referred to by the initials HbA1c. It helps us distinguish a borderline case from a full-blown one.

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