HbA1c (also written A1C) is one of the most common tests in diabetes care. It is a laboratory blood test, not something you check at home with a finger-prick meter.
What it measures
Sugar (glucose) in the blood attaches to haemoglobin, the protein inside red blood cells. The HbA1c test measures how much haemoglobin has glucose attached. Because red blood cells are replaced roughly every three months, the result reflects your average blood sugar over about the past 2 to 3 months, as the American Diabetes Association (ADA) and the US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) both explain.
You do not need to fast for it. The NIDDK notes the blood can be taken at any time of day.
How the number is read
For diagnosis, the ADA uses these bands:
- Below 5.7% — normal
- 5.7% to 6.4% — prediabetes
- 6.5% or higher — diabetes
The ADA also notes that a diagnosis usually needs the test repeated on a second day. One result on its own is not the final word.
For most people living with diabetes, the ADA’s recommended target is an HbA1c of 7% or less. Your doctor may set a different target for you, and theirs is the one that counts.
The ADA publishes a conversion from HbA1c to an “estimated average glucose”: an HbA1c of 7%, for example, corresponds to an average of about 154 mg/dL.
What it cannot tell you
HbA1c is an average. It does not show the highs after meals or the lows at night, and quite different day-to-day patterns can produce the same number. That is why the readings you take at home matter alongside it.
When the result can mislead
The NIDDK lists situations in which HbA1c can be falsely high or low, including:
- iron-deficiency anaemia, which can make the result falsely high;
- kidney failure or liver disease;
- recent blood loss, a blood transfusion, dialysis, or treatment with erythropoietin;
- inherited haemoglobin variants — for example in families where thalassaemia or sickle-cell disease runs.
If any of these apply to you, tell your doctor. They may use other tests alongside HbA1c.
ⓘThis is general information, not medical advice. Talk to your doctor.
Sources
- American Diabetes Association — “Understanding A1C”
- American Diabetes Association — “Diagnosis”
- NIDDK (US National Institutes of Health) — “The A1C Test & Diabetes”
Last reviewed 1 October 2026.