Hemoglobin A1c
The percentage of hemoglobin with glucose attached, reflecting average blood sugar over about 3 months.
| Typical reference range | below 5.6 % |
| Commonly cited optimal range | below 5.4 % |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Glucose attaches irreversibly to hemoglobin in red blood cells. Because red cells live about 120 days, HbA1c reflects average blood glucose over the previous 2-3 months, weighted toward the most recent weeks.
Why it matters
HbA1c is used to diagnose prediabetes (5.7-6.4%) and diabetes (6.5% or higher) and to monitor glucose control. Even within the normal range, higher values are associated with greater cardiovascular risk.
What a high result can mean
Elevated values indicate chronically raised blood glucose from insulin resistance, prediabetes or diabetes. Iron deficiency, B12/folate deficiency, kidney failure, splenectomy and older red cell populations can falsely raise HbA1c.
What a low result can mean
Low values are uncommon; falsely low results occur when red cells turn over quickly, for example with hemolysis, recent blood loss or transfusion, pregnancy, or treatment of anemia. Certain hemoglobin variants affect some assays.
What affects the result
No fasting required. Values in mmol/mol (IFCC) relate to % by a non-proportional formula (% = 0.0915 x mmol/mol + 2.15), so enter values in %. Because it depends on red cell lifespan, compare with fasting glucose, fructosamine or CGM data if results seem inconsistent. Exercise, weight loss and dietary changes lower it over 2-3 months.
Unit
Usually reported in %.
Also called on lab reports
HbA1c, A1c, Hgb A1c, Glycated Hemoglobin, Glycohemoglobin, Glycosylated Hemoglobin, HbA1c (NGSP)
Related tests: Glucose & Metabolic
This page is educational and does not replace advice from a healthcare professional.