Glycated hemoglobin (HbA1c) is the fraction of the hemoglobin in your red blood cells that has glucose chemically stuck to it, which makes it a running average of blood sugar over the previous two to three months. Because the sugar attaches slowly and stays attached for the life of the cell, the number cannot be talked down by skipping breakfast or fixed by one good week. It is the standard test for diagnosing and monitoring type 2 diabetes, and one of the few routine blood markers that tracks a long-run habit rather than a moment.
What it measures
Glucose circulating in the blood binds to hemoglobin without any help from enzymes. The higher the glucose, the more hemoglobin ends up glycated. Red cells survive roughly four months, so the measured fraction reflects average exposure over that window, weighted toward the most recent weeks. The test is a single blood draw, needs no fasting, and can be taken at any time of day. That is the practical difference from Blood glucose (fasting), which is a snapshot of one morning and moves with sleep, stress and last night’s dinner.
Results are reported two ways: as a percentage under the NGSP scale, common in the United States, and as mmol/mol under the IFCC scale used across much of Europe. Many labs print both, plus an estimated average glucose derived from the result. The diagnostic bands, the assay caveats and the comparison with fasting glucose are set out in more depth in Glycated hemoglobin (HbA1c).
Typical values
| HbA1c | IFCC equivalent | Usual interpretation |
|---|---|---|
| Below 5.7% | Below 39 mmol/mol | Normal range |
| 5.7-6.4% | 39-46 mmol/mol | Intermediate band, above normal but below diabetes |
| 6.5% and above | 48 mmol/mol and above | Diabetes range; diagnosis normally needs confirmation |
The 6.5% diabetes threshold is consistent across major guidelines. The intermediate band, usually called Prediabetes, is not: some bodies start it at 5.7%, others only at 6.0%, so the same result can be labeled differently depending on which guideline your lab follows. The test also misleads when red cell turnover is abnormal. Recent blood loss, hemolysis, a transfusion, pregnancy, advanced kidney disease, iron deficiency and inherited hemoglobin variants can all push the figure up or down independently of your actual glucose, and some assays handle variants better than others. In those situations a clinician will lean on glucose measurements instead.
Why it matters for longevity
In large observational cohorts, higher HbA1c is associated with more cardiovascular disease, kidney disease, dementia and all-cause mortality, and the association is visible well below the diabetes cut-off. Several cohorts also report a J-shaped curve, with slightly higher mortality at the low end, which is widely thought to reflect underlying illness, frailty or aggressive drug treatment rather than a harm of low blood sugar itself. Trials tell a narrower story than the cohorts do: lowering HbA1c in people with diabetes clearly reduces microvascular damage to eyes, nerves and kidneys, while the effect on total mortality is smaller and less consistent. Nobody has run a trial testing whether nudging a normal HbA1c down further extends life.
What changes it
The levers are the familiar metabolic ones. Losing excess weight moves it most reliably. Cutting refined carbohydrate and sugary drinks lowers the glucose peaks that drive glycation. Structured exercise programs reduce HbA1c meaningfully in people with type 2 diabetes, with aerobic and resistance training both working and the combination working best; a ten to fifteen minute walk after meals blunts the post-meal rise. Short or broken sleep raises glucose and undoes some of that. Metformin, GLP-1 receptor agonists and other glucose-lowering drugs act directly. Whatever you change, expect to wait about three months before a repeat test shows it, because the marker is an average and averages move slowly.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- How to Improve Metabolic Health: A Practical Guide
- Heart Health: A Prevention Guide for Longevity
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.