Key takeaways
- HbA1c gives an average of your blood sugar over the past 2 to 3 months, reported in mmol/mol.
- NICE and NHS guidance sort results into three established bands: normal, prediabetes and the diabetes range.
- If you have no symptoms, a result of 48 mmol/mol or above is confirmed with a repeat test before diabetes is diagnosed.
- Anaemia, certain haemoglobin variants and pregnancy can affect the accuracy of HbA1c, so your doctor weighs these alongside the number.
- Because it reflects an average, you can eat and drink normally before the test.
A routine check has come back with HbA1c on it and a number in mmol/mol, or a doctor has mentioned your blood sugar might be running a little high. Either way, you'd like to know what the number really says.
HbA1c measures the proportion of sugar attached to your red blood cells. That gives an average picture of your blood sugar over the past 2 to 3 months, rather than a single reading. NICE and NHS guidance use established thresholds: below 42 mmol/mol is normal, 42 to 47 is prediabetes, and 48 or above is in the diabetes range. You can book an HbA1c blood test at Blood London without a GP referral.
Book an HbA1c blood test at 117a Harley Street: £71.25 plus the £50 blood draw, results back in 1 day, and no fasting needed.
What does HbA1c actually measure?
Haemoglobin is the protein in red blood cells that carries oxygen, and HbA1c is the percentage of it that has sugar attached. Red blood cells last around 2 to 3 months, so the result reflects your average blood sugar over that whole period rather than what you ate that morning.
That's the key difference from a fasting glucose test, which shows your blood sugar only at the moment of the sample.
What do the different HbA1c results mean?
NICE and NHS guidance set out three bands. Below 42 mmol/mol is considered normal. A result of 42 to 47 mmol/mol is prediabetes, sometimes called non-diabetic hyperglycaemia. It means your blood sugar is higher than normal but not yet in the diabetes range. A result of 48 mmol/mol or above is in the diabetes range.
If you have no symptoms, the result is confirmed with a second test before diabetes is diagnosed. If you have typical symptoms, a doctor may diagnose it from one result.
| HbA1c result | What it generally means |
|---|---|
| Below 42 mmol/mol | Normal |
| 42 to 47 mmol/mol | Prediabetes (non-diabetic hyperglycaemia) |
| 48 mmol/mol or above | Diabetes range (confirmed by a repeat test if you have no symptoms) |
These are NICE and NHS established thresholds. Your doctor reads the result alongside your symptoms and history.
Do I need to fast before an HbA1c test?
No. HbA1c reflects an average over months rather than your blood sugar at that moment, so you can eat and drink normally beforehand. A fasting glucose test is different and does need preparation. If your doctor has ordered both together, ask which parts need fasting.
Can other health conditions affect my HbA1c result?
Yes. The test relies on the normal lifespan and structure of red blood cells, so anaemia and certain haemoglobin variants can affect its accuracy. Recent blood loss or a blood transfusion, kidney disease and some medicines can also affect it.
Pregnancy changes the result too, and diabetes in pregnancy is usually assessed with a glucose tolerance test instead. If any of these apply to you, your doctor takes it into account when reading your HbA1c.
What happens if my HbA1c is raised?
If you have no symptoms, a single raised result is confirmed with a repeat test before diabetes is diagnosed. Your doctor will also consider your symptoms, weight, family history and other risk factors.
A result in the prediabetes range is usually a prompt for lifestyle changes and closer monitoring, which is worth discussing with your doctor.
If you are very thirsty, passing a lot of urine, losing weight without trying, or feel very unwell, drowsy or are vomiting, get medical advice the same day through your GP or NHS 111. Call 999 if you are seriously unwell.
Which tests look at diabetes risk in more detail?
If you'd like more than HbA1c alone, these panels go further. Each is priced plus the £50 blood draw.
- Diabetic Profile 1 pairs HbA1c with a fasting glucose reading, giving both a snapshot and a trend from one visit: £83.75, results in 1 day. The glucose part needs an 8-hour fast.
- The insulin resistance blood test adds a fasting insulin resistance index alongside HbA1c and a lipid profile: £279, results in 1 working day, with a 10 to 12 hour fast.
- The complete metabolic blood test is the broadest single-visit panel, adding thyroid, liver, kidney and uric acid markers: £675, results in up to 9 working days, with a 10 to 12 hour fast.
If you'd like a doctor to go through your result, you can add a doctor review to your test for £75. To talk about what type 2 diabetes means for you specifically, your GP can look at your whole picture alongside the blood result.
How HbA1c testing works at our Harley Street clinic
HbA1c is one of our most popular private blood tests in London. No fasting is needed, and results are sent by email, usually within 1 day. You can walk in or book online at 117a Harley Street, seven days a week, with no GP referral. Start your booking.
If you'd like these markers alongside a doctor's examination and a wider health assessment, the Full Body MOT health check includes HbA1c as part of a broader review.
Frequently asked questions
Is HbA1c the same as a blood sugar test?
Not quite. A standard glucose test shows your blood sugar at the moment of the sample, while HbA1c gives an average over the past 2 to 3 months, so the two answer slightly different questions.
Can I have a normal HbA1c and still have diabetes?
It's uncommon but possible. A condition affecting red blood cells can make the result less reliable, or blood sugar may have risen quickly. That's one reason your doctor considers your symptoms and other risk factors too.
How quickly does HbA1c change?
Because it reflects an average over 2 to 3 months, HbA1c changes more slowly than day-to-day blood sugar. It won't show short-term changes, such as what you ate that day.
Does HbA1c diagnose type 1 diabetes?
HbA1c is mainly used to assess type 2 diabetes and prediabetes. Type 1 diabetes can develop quickly, so HbA1c may not yet be raised. If you have symptoms such as thirst, passing a lot of urine and weight loss, see a doctor the same day.
Do I need to repeat my HbA1c test if it comes back raised?
Usually, yes. If you have no symptoms, a result in the diabetes range is confirmed with a repeat test before diabetes is diagnosed.
Will losing weight lower my HbA1c?
For many people, weight loss, diet changes and more activity can improve HbA1c over time, particularly in prediabetes. Your doctor can tell you whether that's enough on its own or whether treatment is needed too.
References
- Type 2 diabetes in adults: management (NICE)
- Type 2 diabetes (NHS)
- Diabetes, type 2 (NICE CKS)
- HbA1c (Diabetes UK)