What Is HbA1c, and What Counts as Normal?
A doctor explains what HbA1c measures, the numbers that define normal, prediabetes and diabetes, and the situations where the test can mislead.
The short version
- HbA1c reflects your average blood glucose over roughly the past two to three months, measured as the proportion of hemoglobin that has sugar attached to it.
- Normal is below 42 mmol/mol (6.0 per cent), prediabetes is 42 to 47 mmol/mol (6.0 to 6.4 per cent), and diabetes is 48 mmol/mol (6.5 per cent) or above on repeated testing.
- The test is unreliable in anemia, recent blood loss, pregnancy and some hemoglobin variants, where glucose testing is used instead.
See a doctor promptly if
These are the signs that change this from something to read about into something to act on.
- HbA1c in the diabetes range alongside heavy thirst, frequent urination, weight loss or blurred vision - arrange review promptly
- Vomiting, deep rapid breathing, abdominal pain or drowsiness in someone with diabetes - seek emergency care
- Repeated low blood glucose episodes despite a good HbA1c
HbA1c measures the proportion of the hemoglobin in your red blood cells that has glucose stuck to it. Because red cells live about 120 days, that proportion reflects your average blood glucose over roughly the last two to three months, weighted towards the most recent six weeks.
That is what makes it useful. A finger-prick glucose tells you about one moment; HbA1c tells you about a season.
The numbers#
| HbA1c | Interpretation |
|---|---|
| Below 42 mmol/mol (below 6.0 per cent) | Normal |
| 42 to 47 mmol/mol (6.0 to 6.4 per cent) | Prediabetes, or high risk |
| 48 mmol/mol (6.5 per cent) or above | Diabetes range |
One result in the diabetes range is not usually enough on its own. Unless you have clear symptoms such as thirst, frequent urination and weight loss, the diagnosis is normally confirmed with a second test. Different countries use slightly different cut-offs for the prediabetes band, with some starting at 39 mmol/mol (5.7 per cent), so read your result against your local reference range.
Why the prediabetes band matters#
That middle band is not a diagnosis; it is a warning with time attached. A substantial proportion of people in that range progress to type 2 diabetes over the following years, and a substantial proportion do not, particularly those who change activity, weight and diet in the meantime. Structured lifestyle programs have some of the strongest evidence in preventive medicine.
When HbA1c misleads#
The test assumes normal red cells with a normal lifespan. When that assumption breaks, the number is wrong in predictable directions.
It reads falsely low when red cells are short-lived or being rapidly replaced: haemolytic anemia, recent significant blood loss, recent transfusion, pregnancy, advanced kidney or liver disease, and some sickle cell and thalassaemia traits.
It reads falsely high when red cells are long-lived: iron deficiency anemia before treatment, B12 or folate deficiency, and after splenectomy.
In these situations a doctor will rely on fasting glucose, an oral glucose tolerance test, or continuous glucose data instead. This is a common reason for two apparently contradictory results in the same person.
What actually moves the number#
Consistent changes over months, not heroic weeks. Regular activity, particularly after meals, improves insulin sensitivity measurably. Reducing refined carbohydrate and sugary drinks lowers average glucose. Modest weight loss of five to ten per cent has a disproportionately large effect for people carrying weight around the middle. Sleep and stress matter more than most people expect.
If you take glucose-lowering medicines, do not adjust them yourself in response to a result. Bring the number to whoever prescribes them.
The wider picture#
If your result landed in the prediabetes band, the in-depth guide to prediabetes and insulin resistance explains what is happening physiologically and which changes have the best evidence behind them.
Common questions
Do I need to fast for an HbA1c?
How quickly can HbA1c change?
What are the two different units?
Sources
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