HbA1c measures the percentage of hemoglobin molecules with glucose attached via non-enzymatic glycation. Reflects average blood glucose over the prior 8-12 weeks (the lifespan of red blood cells).
Mechanism — Higher and more sustained blood glucose produces more glycation. Single value smooths over daily glucose noise.
Lab range — Optimal under 5.4%. Pre-diabetic: 5.7-6.4%. Diabetic: greater than or equal to 6.5%. For longevity-minded individuals, target 5.0-5.4%.
Caveats — Anemia, recent blood loss, or hemoglobinopathies can give falsely low or high values. Pair with fasting glucose and fasting insulin for a fuller picture. CGM data exposes hypoglycemia or peak excursions that A1c hides.