What the A1c test measures

Glucose sticks to hemoglobin

Hemoglobin is the protein in red blood cells that carries oxygen. Glucose in the blood attaches to it, and the more glucose there is, the more attaches. The A1c test measures the share of hemoglobin with glucose attached (NIDDK).

The test has other names: hemoglobin A1c, HbA1c, glycated hemoglobin and glycohemoglobin (NIDDK).

Why about 3 months

Red blood cells live for a few months, so the result reflects glucose over roughly that period. The most recent 30 days count more than the earlier months, and large changes over the past month will show up in the result. Short spikes and dips do not (NIDDK). See how fast A1c can change for a week-by-week view.

How the result is reported

• In the US, A1c is reported as a percentage. A normal A1c is below 5.7% (NIDDK).
• The ADA uses 5.7% to 6.4% for prediabetes and 6.5% or higher for diabetes (ADA Standards 2026, sec. 2).
• Many labs outside the US report A1c in mmol/mol (IFCC units). The converter handles both.
• Some labs also report an estimated average glucose (eAG), calculated from the A1c with the ADAG formula (Nathan 2008).

No fasting needed

Unlike a fasting glucose test, you do not need to fast before an A1c test, so blood can be drawn at any time of day (NIDDK).

How often

For people with diabetes, experts recommend an A1c test at least twice a year, and more often if they are not meeting their goal (NIDDK).

Next: A1c, fasting glucose and GMI.