What does HbA1c measure?

HbA1c measures glycated hemoglobin. Because red blood cells circulate for weeks, it reflects a longer window than a single glucose measurement, but it does not show day-to-day highs and lows. ^{1, 2}

What the result can—and cannot—tell you

It can help show

A longer-term estimate of glucose exposure ^{1, 2}
A trend over time ^{1, 2}
Information used in diabetes screening and monitoring ^{1, 2}

It cannot show by itself

Your glucose at the moment of the blood draw ^{1, 2}
Every glucose spike or low ¹
A reliable result in every condition that changes red blood cell turnover ^{1, 2}

Reference range vs. clinical decision

Use the range on your own laboratory report.

Diagnostic cutoffs are not the same thing as a laboratory reference interval or a personal treatment target. A clinician may repeat or confirm an abnormal result, and pregnancy, anemia, hemoglobin variants, kidney disease, blood loss or transfusion can make HbA1c less reliable. ^{1, 2}

Context that can change interpretation

Conditions that shorten or lengthen red blood cell survival ^{1, 2}
Pregnancy and recent blood loss or transfusion ²
Hemoglobin variants and some kidney or liver conditions ²

Preparing for the test

HbA1c usually does not require fasting, though another test ordered at the same visit might. Follow the instructions supplied with your order. ¹

Common questions

Is HbA1c the same as a fasting glucose test?

No. Fasting glucose is a point-in-time measurement. HbA1c estimates average exposure over a longer period. ^{1, 2}

Can one HbA1c result diagnose diabetes?

Sometimes confirmation is needed, especially without classic symptoms. Interpretation belongs with a qualified clinician. ²

Sources and evidence notes

Hemoglobin A1C (HbA1c) TestMedlinePlus · 2025
Patient-oriented overview, uses and limitations.
The A1C Test & DiabetesNational Institute of Diabetes and Digestive and Kidney Diseases · 2024
Federal guidance on interpretation and factors that affect accuracy.

Revision history

2026-08-17
Initial evidence-led guide and source map published. — Syed Jafri

Hemoglobin A1c (HbA1c)