An HbA1c blood test shows your average blood sugar (glucose) level over the past 2 to 3 months (Diabetes UK). In the UK, a result below 42 mmol/mol is in the normal range, 42–47 mmol/mol means a higher risk of type 2 diabetes (often called prediabetes), and 48 mmol/mol or above suggests diabetes (NICE PH38).
Key takeaways
- HbA1c reflects your average blood sugar over 2–3 months, and you don't need to fast for it (Lab Tests Online UK).
- Below 42 mmol/mol is normal, 42–47 is high risk (prediabetes, or non-diabetic hyperglycaemia) and 48 or above suggests diabetes (NICE PH38).
- Without symptoms, a result of 48 or above is repeated to confirm a diagnosis (Diabetes UK).
- HbA1c can be misleading in anaemia, pregnancy, some haemoglobin variants and after a recent blood transfusion (Lab Tests Online UK).
What HbA1c measures
Glucose in your blood sticks to haemoglobin, the protein in red blood cells. Because red blood cells live for about 2 to 3 months, measuring this "glycated haemoglobin" gives an average picture of your blood sugar over that time, rather than a snapshot of one moment (Diabetes UK).
That's why it's useful for spotting type 2 diabetes and prediabetes, and for checking how well diabetes is managed. It's a normal blood sample from a vein, and no preparation is needed (Lab Tests Online UK).
Results are given in mmol/mol, the UK standard since 2009, though you may still see the older percentage figure (Diabetes UK).
HbA1c ranges explained
| HbA1c (mmol/mol) | HbA1c (%) | What it means |
|---|---|---|
| Below 42 | Below 6.0% | Normal range |
| 42–47 | 6.0–6.4% | High risk of type 2 diabetes (prediabetes, or non-diabetic hyperglycaemia) |
| 48 or above | 6.5% or above | Suggests diabetes; confirmed with a repeat test if you have no symptoms |
Sources: NICE PH38; Diabetes UK; Diabetes UK, on prediabetes.
A few points worth knowing:
- A normal HbA1c doesn't always rule diabetes out. A result below 48 doesn't exclude a diagnosis made with glucose tests (Diabetes UK).
- Below 42 isn't a free pass if you have other risk factors. NICE describes people with a high risk score but an HbA1c below 42 as at moderate risk (NICE PH38).
- If you already have diabetes, your target is agreed with your care team. Diabetes UK describes an ideal of 48 mmol/mol or below for many people (Diabetes UK).
What happens if your HbA1c is 42–47
A result in this range is a warning sign, and a chance to act. NICE recommends that people at high risk are referred to an intensive lifestyle-change programme, and have a blood test at least once a year (NICE PH38). In England, your GP practice may refer you to the Healthier You NHS Diabetes Prevention Programme, a nine-month lifestyle-change programme (NHS England).
Eating well, moving more and, if you need to, losing some weight are the main levers. Our guides to how much weight you can lose in a month and blood test results and preparation may help.
When HbA1c can be inaccurate
HbA1c depends on healthy red blood cells, so anything that changes them can skew the result (Lab Tests Online UK; Diabetes UK):
| Situation | Possible effect |
|---|---|
| Iron deficiency | May raise HbA1c |
| Haemolytic anaemia or heavy bleeding | May give a falsely low result |
| Recent blood transfusion | Not reliable for diagnosis |
| Haemoglobin variants, such as sickle cell or thalassaemia | Accuracy depends on the lab method |
| Pregnancy | Not used to diagnose diabetes in pregnancy |
HbA1c also isn't used to diagnose diabetes in children and young people, in suspected type 1 diabetes, or when symptoms have lasted less than 2 months (Diabetes UK). In these cases, glucose tests are used instead.
HbA1c and the NHS Health Check
If you're aged 40 to 74 and don't have conditions such as heart disease, diabetes or high blood pressure, you should be invited for a free NHS Health Check every 5 years. It takes about 20 to 30 minutes and covers your weight, waist, blood pressure and cholesterol, and may include a blood sugar test (NHS). If you haven't been invited, contact your GP practice.
Renvo can help. If you'd like blood tests outside the NHS, our London clinic offers them in person, and a clinician explains what your results mean and what, if anything, to do next. blood tests at our London clinic
When to see a GP
See a GP if you have possible symptoms of type 2 diabetes, such as (NHS):
- feeling very tired, thirsty all the time or peeing more than usual
- losing weight without trying
- blurred vision, cuts that heal slowly, or repeated thrush
Ask for an urgent GP appointment or contact NHS 111 if you think you or your child may have type 1 diabetes. Call 999 or go to A&E if thirst, peeing a lot and tiredness come with being sick, stomach pain, fast deep breathing, or feeling sleepy or confused, as these can be signs of diabetic ketoacidosis (NHS).
For what a wider panel costs and includes, see our guide to private blood tests in the UK.
See also our guide to IV drips in the UK.
Honest FAQs
Do I need to fast for an HbA1c blood test?
No. HbA1c reflects your average blood sugar over 2–3 months, so no preparation is needed (Lab Tests Online UK).
What is a normal HbA1c level?
Below 42 mmol/mol (6.0%). A result of 42–47 means a higher risk of type 2 diabetes, and 48 or above suggests diabetes (NICE PH38).
How often should HbA1c be checked?
If you're at high risk, NICE recommends a blood test at least once a year. People with diabetes are usually tested every 3 to 6 months (NICE PH38; Diabetes UK).
Is prediabetes the same as non-diabetic hyperglycaemia?
Yes. Diabetes UK uses the two terms together for blood sugar that's higher than normal but not in the diabetes range. For HbA1c, NICE defines this high-risk range as 42–47 mmol/mol (Diabetes UK; NICE PH38).
This article is general information, not medical advice. Always speak to a qualified clinician about your own symptoms.
