In short
HbA1c (hemoglobin A1c) is a blood test that measures the percentage of hemoglobin that has become glycated (bound to glucose), reflecting average blood glucose levels over roughly the preceding two to three months, matching the lifespan of a red blood cell.
Glucose in the bloodstream binds non-enzymatically to hemoglobin inside red blood cells at a rate roughly proportional to average blood glucose concentration over the cell's circulating lifespan of about 120 days, though the most recent 30 days contribute disproportionately to the measured result. This makes HbA1c a longer-term integrated measure of glucose exposure, in contrast to a fasting glucose test, which reflects blood sugar at a single point in time.
The American Diabetes Association uses HbA1c as a standard diagnostic criterion: below 5.7% is normal, 5.7–6.4% indicates prediabetes, and 6.5% or above on two separate tests indicates diabetes. It is also used to monitor glycemic control in people already diagnosed with diabetes, with a common treatment target of below 7% for many adults, individualized based on age and comorbidities.
HbA1c has known limitations: any condition that changes red blood cell turnover — including anemia, recent blood loss or transfusion, pregnancy, and certain hemoglobin variants — can produce a falsely low or high result independent of actual average glucose, which is why clinicians cross-check an unexpected HbA1c result against fasting glucose or an oral glucose tolerance test rather than relying on it in isolation in those situations.