What it means
A measure of how much sugar has attached to your red blood cells, which reflects your average blood glucose over roughly three months. Reported in mmol/mol in the UK.
- Below 42 — normal
- 42 to 47 — prediabetes, sometimes called non-diabetic hyperglycaemia
- 48 or above — in the diabetes range, usually needing a second confirming test
Because red cells live about three months, it cannot be improved by being careful for a week beforehand — which is exactly what makes it useful.
In practice
On health screens, NHS Health Checks, and every diabetes review.
It is also used to monitor treatment once diabetes is diagnosed, with an individual target agreed rather than one figure for everybody.
Why it matters
If you are in the 42 to 47 range, ask to be referred to the NHS Diabetes Prevention Programme. It is free, and it roughly halves progression to type 2 diabetes in people who complete it.
This is the single most actionable abnormal result on a routine panel, and it is frequently reported without anything being offered.
A result of 48 or more usually needs repeating to confirm, unless you have clear symptoms.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026