What Is an A1C Test? A Plain-English Guide

Reviewed for accuracy against named public sources. Educational content only — see our Medical Disclaimer.

If your doctor has ordered an A1C test, you may be wondering what it actually measures — and how it's different from the number on a glucose meter. Here's the short version: A1C looks backward over months, while a fingerstick or continuous glucose monitor (CGM) reading is a single snapshot in time.

What A1C actually measures

Glucose in your bloodstream attaches to hemoglobin, the protein in red blood cells that carries oxygen. The more glucose that has been circulating, the more of it sticks to hemoglobin. Because red blood cells live for about three months, an A1C test estimates the average percentage of hemoglobin that's carrying glucose over that window — giving your care team a longer-term picture than any single blood draw could.

How the result is reported

A1C is reported as a percentage. According to the American Diabetes Association, the general categories are:

Below 5.7%: considered in the normal range
5.7% to 6.4%: consistent with prediabetes
6.5% or higher (on two separate tests): consistent with diabetes

These cutoffs come from population-level research and are meant to guide further testing and conversation with your provider — not to serve as a standalone diagnosis.

Turning A1C into a number you recognize

Because A1C is a percentage rather than a glucose reading, it can be hard to connect it to the mg/dL or mmol/L numbers you might see on a meter. That's what an estimated average glucose (eAG) conversion is for — it translates your A1C into the same units your glucose meter uses, so the two numbers are easier to compare side by side. Try it with our A1C to eAG converter.

What A1C doesn't tell you

A1C is an average, so it can't show you day-to-day swings, or distinguish between someone whose glucose is fairly steady versus someone who alternates between highs and lows that average out to the same number. That's one reason your care team may also look at fasting glucose or CGM data alongside A1C — see fasting glucose vs. A1C for how these tests complement each other.

How often is it checked?

For people managing diabetes, A1C is commonly rechecked every 3 to 6 months, though this varies by individual and how stable your levels are. Read more in how often should you test your blood sugar.

What can affect A1C accuracy

A1C is generally reliable, but a handful of conditions can skew results independent of actual blood sugar. Certain forms of anemia, other blood disorders affecting red blood cell lifespan, recent significant blood loss, and some kidney or liver conditions can all push A1C higher or lower than glucose levels alone would suggest. If your A1C result doesn't seem to match your day-to-day glucose readings, this is worth raising with your doctor rather than assuming one number or the other is wrong.

What to bring to your next appointment

If you're getting an A1C test, it can help to bring a log of recent glucose readings if you track them, a list of current medications, and any questions about how the result compares to your personal targets. Because A1C reflects an average, it's also worth mentioning any recent illness, major stress, or significant diet changes, since these can shift the number for reasons unrelated to your usual routine.

Sources

NIDDK: The A1C Test & Diabetes · American Diabetes Association: Understanding A1C