HbA1c Test
What HbA1c measures, the thresholds for diabetes and prediabetes, and when it is unreliable.
Last revised
A blood test reflecting average blood glucose over the preceding two to three months. It requires no fasting and is the standard test for diagnosing and monitoring type 2 diabetes in the UK.
What it actually measures
Glucose attaches to haemoglobin in red blood cells, and the proportion that has been modified reflects average glucose over the lifespan of those cells, roughly 120 days. Because it is an average, it smooths out day-to-day fluctuation — which is its strength for diagnosis and its limitation for spotting variability.
The thresholds
In the UK, HbA1c is reported in mmol/mol. A result of 48 mmol/mol (6.5 per cent) or above is diagnostic of diabetes, requiring confirmation on a second sample if there are no symptoms. Between 42 and 47 indicates non-diabetic hyperglycaemia, often called prediabetes, and is the point at which structured prevention programmes are offered. Below 42 is normal.
When it is not reliable
Anything that changes red cell lifespan distorts the result. It is unreliable in haemoglobinopathies such as sickle cell disease and thalassaemia, in recent significant blood loss or transfusion, in haemolytic anaemia, in advanced kidney disease, in pregnancy, and in iron deficiency anaemia, which can raise it spuriously. In these situations other tests are used.
Targets in established diabetes
A commonly used target is around 48 mmol/mol for people managed by diet or a single medicine, and around 53 where a medicine carrying a risk of low blood sugar is used. Targets are individualised: tighter control benefits younger people with long life expectancy, while looser targets are appropriate where hypoglycaemia risk, frailty or other conditions dominate.
At a glance
| Term | Detail |
|---|---|
| What it reflects | average blood glucose over 2-3 months |
| Diagnostic threshold | 48 mmol/mol (6.5%) or above |
| Non-diabetic hyperglycaemia | 42-47 mmol/mol |
| Normal | below 42 mmol/mol |
| Fasting required | no |
| Common target on diet or one medicine | around 48 mmol/mol |
| Unreliable in | haemoglobinopathies, pregnancy, recent blood loss, advanced kidney disease |
When to get help
Seek urgent medical help for excessive thirst and urination with vomiting, abdominal pain, deep or rapid breathing, breath smelling of pear drops, or drowsiness — this can indicate diabetic ketoacidosis. If you take insulin or a sulfonylurea, treat symptoms of low blood sugar immediately with fast-acting carbohydrate, and seek help for repeated episodes or any loss of consciousness.
Where to go next
Where this comes from
- NICE - Type 2 diabetes in adults: management
- NHS - HbA1c test
- Diabetes UK - understanding HbA1c
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
