HbA1c (hemoglobin A1C)

How it works

Glucose in your blood sticks to hemoglobin, the oxygen-carrying protein in red blood cells. The higher your blood sugar runs, the more hemoglobin becomes 'glycated.' Since a red blood cell lasts roughly three months, the A1C captures a rolling average, with the most recent weeks counting a little more than earlier ones.

You don't need to fast, which makes A1C convenient to add to routine blood work.

The diagnostic cutoffs

These thresholds are used for adults in the U.S.:

• Below 5.7%: normal.
• 5.7% to 6.4%: prediabetes.
• 6.5% or higher: diabetes, usually confirmed with a second test on another day unless symptoms are obvious.

When A1C can mislead

Anything that changes how long red blood cells survive, or how hemoglobin behaves, can push the result off. Watch for:

• Iron deficiency, which can falsely raise A1C.
• Recent blood loss or transfusion, hemolysis, kidney failure, dialysis or erythropoietin treatment.
• Liver disease and some medicines, including certain HIV drugs and opioids.
• Hemoglobin variants such as sickle cell trait, which are more common in people of African, Mediterranean or Southeast Asian ancestry.
• Pregnancy, especially early and late.

What A1C doesn't show

An average can hide swings. Two people with the same A1C can have very different days: one steady, one with large spikes after meals and dips overnight. Continuous glucose monitoring or fasting glucose can fill in that detail. If your A1C and glucose readings don't match, that mismatch is itself a clue worth raising with your clinician.

A1C also isn't used to diagnose type 1 diabetes or gestational diabetes. And because it's a percentage of hemoglobin, it can't tell you whether a high reading came from breakfast, late-night snacking or a stretch of poor sleep; daily patterns need daily data.

Using it over time

Many adults have an A1C checked as part of screening, and people with prediabetes or diabetes typically repeat it at least twice a year. Movement over time is often more useful than a single number. Weight loss, physical activity, changes in diet and medications such as metformin or GLP-1 receptor agonists can all bring A1C down; targets for people with diabetes are set individually, often around 7% or less.

If you're starting a GLP-1 medication or another treatment that affects blood sugar, a baseline A1C gives you and your clinician a reference point, and a repeat test after about three months shows the effect.