What does HbA1c measure?
Glucose in your blood sticks to a protein in red blood cells called haemoglobin. The more glucose there has been, the more is stuck. Because red blood cells live about three months, HbA1c reflects your average glucose over that time — not just on the day of the test.
In the UK it is reported in mmol/mol; you may also see an older percentage (%) value.
What do the numbers mean?
As a general guide, a result of 48 mmol/mol (6.5%) or above on testing is used to diagnose diabetes, and 42–47 mmol/mol (6.0–6.4%) suggests a higher risk of developing diabetes (sometimes called 'pre-diabetes').
For people who already have diabetes, a common target is around 48–58 mmol/mol, but the right target is individual — your team will set one that balances good control against the risk of low blood sugar.
What can affect the result?
HbA1c is not reliable in everyone. Conditions that change red blood cells — such as anaemia, recent blood loss or transfusion, pregnancy, and some inherited conditions — can make it falsely high or low. In those situations other tests are used.
Unlike a finger-prick glucose test, you do not usually need to fast for an HbA1c blood test.
Common questions
Is HbA1c the same as a finger-prick glucose test?
No. A finger-prick shows your glucose at that moment. HbA1c shows the average over the past few months, so it is less affected by what you ate that day.
Can I lower my HbA1c?
Often yes — through diet, physical activity, weight management and, where needed, medication. Any changes should be planned with your diabetes team, who can monitor progress and adjust treatment safely.
Related tools
These calculators are designed for healthcare professionals.