A1C (glycated hemoglobin)

A1C, or glycated hemoglobin, is a blood test that reflects your average blood glucose over roughly the past three months. Glucose in the bloodstream attaches to hemoglobin in red blood cells, and because those cells live about three months, the percentage of hemoglobin carrying attached glucose becomes a running average. NIDDK reports a normal A1C as below 5.7 percent.

Unlike a fingerstick reading, A1C cannot be gamed by skipping breakfast. That is why it is the primary test used both to diagnose type 2 diabetes and prediabetes and to manage diabetes after diagnosis. It is also the endpoint most diabetes drug trials are built around.

For GLP-1 medications the effect is substantial. In SURPASS-2, A1C fell by 2.01, 2.24 and 2.30 percentage points on tirzepatide 5, 10 and 15 mg over 40 weeks, against 1.86 points on semaglutide 1 mg. Moving someone from 9% to below 7% is the difference between poorly controlled diabetes and controlled diabetes, and it is achieved partly through direct glucose-dependent insulin release and partly through weight loss.

Two confusions are common. First, A1C is not a weight measure — it is entirely possible to improve A1C substantially with modest weight loss, and the two do not move in lockstep. Second, A1C can read falsely high or low in conditions that change red blood cell lifespan, including anemia, recent blood loss, pregnancy and some hemoglobin variants, which is why a surprising result deserves a repeat rather than an immediate diagnosis.

If you are prescribed a GLP-1, your A1C matters for two separate reasons: it may determine whether your prescription is on-label, and it determines whether adding the drug to insulin or a sulfonylurea creates a hypoglycemia risk that requires those doses to be reduced. Ask about both. Our metabolic marker guide and interaction checker cover the second question.