HbA1c or fasting glucose: which blood sugar test do you need?
Short answer: HbA1c reflects your average blood sugar over the past two to three months, while a glucose test is a snapshot of that moment. HbA1c is convenient because you do not need to fast, but it is not reliable for everyone, and a single result does not diagnose diabetes on its own.
What HbA1c measures
The HbA1c test shows the proportion of your haemoglobin that has sugar attached to it. Because red blood cells live for around three months, the result gives an average rather than a single moment. No fasting is needed.
What a glucose test measures
A fasting glucose test measures the sugar in your blood after an overnight fast. It shows how your body is managing sugar at that moment, so it can be affected by recent food, stress or illness.
How results are usually read in the UK
HbA1c: 42 to 47 mmol/mol is generally classed as prediabetes, and 48 mmol/mol or above falls in the diabetes range.
Fasting glucose: 5.5 to 6.9 mmol/L is generally classed as prediabetes, and 7.0 mmol/L or above falls in the diabetes range.
These are general thresholds. A clinician will usually confirm a raised result with a repeat test and consider your symptoms before making a diagnosis. If you want both markers together, the diabetes status profile is designed for that.
When HbA1c can be unreliable
Anaemia, recent blood loss or a recent blood transfusion
Inherited blood conditions such as sickle cell trait or thalassaemia
Pregnancy
Suspected type 1 diabetes or rapidly developing symptoms, where a glucose test is more appropriate
When not to wait for a routine test
If you have excessive thirst, passing urine very often, unexplained weight loss, blurred vision or recurrent infections, speak to your GP promptly rather than waiting for a routine check.
Frequently asked questions
Do I need to fast for an HbA1c test?
No. HbA1c reflects the past few months, so a recent meal does not change it. A fasting glucose test does need fasting.
How often should I have HbA1c checked?
People with a prediabetes result are usually advised to repeat it about once a year. People with diabetes are usually tested every three to six months, as advised by their care team.
Can HbA1c be normal if I have diabetes?
In some situations, yes. Conditions that shorten the life of red blood cells can lower HbA1c, and a very recent rise in blood sugar may not yet show up. That is why symptoms matter as well as numbers.
Can I lower my HbA1c?
Changes to diet, activity and weight can bring it down, and your clinician can advise on a plan that suits you.
Related reading
Clinically reviewed by Zak Toon, ACP, on 15 October 2026.
This article is general information based on UK guidance from NHS and NICE. It is not personal medical advice. If you are worried about your health or have symptoms, speak to your GP or call 111.
