What does hemoglobin A1c measure?
Hemoglobin A1c reflects average blood glucose over the prior ~3 months by measuring the percentage of glycated hemoglobin. It is the standard metric for diagnosing and tracking prediabetes and type 2 diabetes and is read alongside fasting insulin and fasting glucose.
Full answer
Hemoglobin A1c (HbA1c) measures the percentage of red blood cell hemoglobin that has been irreversibly glycated by circulating glucose. Because erythrocytes live roughly 120 days, A1c reflects average glycemia over approximately the prior three months and is the standard metric for diagnosing and monitoring prediabetes (5.7–6.4%) and type 2 diabetes (≥6.5%). It is most informative when read alongside fasting insulin, fasting glucose, and the HOMA-IR derivation — because insulin resistance frequently precedes A1c elevation by years. Conditions that alter red-cell turnover (anemia, hemoglobinopathies, recent blood loss) can distort A1c, which is why the panel is interpreted in clinical context.
Educational. Not a substitute for individualized provider review.
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- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29