HbA1c blood test: what it measures and what can change it
HbA1c shows the average blood sugar of the past 2–3 months. UK laboratories report it in mmol/mol; older reports use the % scale.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
HbA1c
HbA1c shows the average blood sugar of the past 2–3 months. UK laboratories report it in mmol/mol; older reports use the % scale.
It is the usual way to look for type 2 diabetes and for the range just before it, because it needs no fasting.
Fasting glucose
Fasting glucose is the sugar level in blood after at least 8 hours without food.
It is used with HbA1c to look for diabetes and catches a fast rise that HbA1c can lag behind.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
HbA1c
- For people, 42–47 mmol/mol (6.0–6.4%) is the high-risk band: NICE asks the GP to offer a diabetes prevention programme. NICE PH38 (2017) 1.4.1, 1.5.4; NHS Health Check
- 48 mmol/mol (6.5%) and above is the diabetes range; without symptoms guidelines ask for a second test before anyone is given that diagnosis. WHO 2011; NICE PH38 1.5.7; ADA Standards of Care 2026
Fasting glucose
- For people, 5.5–6.9 mmol/L is the high-risk band NICE uses; 7.0 mmol/L and above is the diabetes range and needs a second test. NICE PH38; WHO 2006; ADA Standards of Care 2026
- Any glucose of 11.1 mmol/L or more together with thirst, passing a lot of urine, weight loss or vomiting is assessed the same day. John et al., Use of HbA1c in the diagnosis of diabetes mellitus in the UK, Diabetic Medicine 2012 (PMID 22957983); NHS Health Check best practice guidance (2019), section 6.6; NHS.uk: Symptoms of type 1 diabetes
What can shift the number
Before the test
- Ate in the 8 hours before. Glucose after a meal reads higher than on an empty stomach: a fasting glucose needs 8 hours without food. Lab Tests Online UK: Glucose tests (fasting blood glucose: no food for at least 8 hours)
Now or in the last 3 months
- Pregnant. In pregnancy the usual ranges for many tests are different: show these results to your midwife or GP. NICE NG201 Antenatal care (2021); NICE NG3 Diabetes in pregnancy (2015, updated 2020); RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NICE NG239 (2024) rec 1.3.2; BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Lipid profile; C-reactive protein (CRP); ESR
- Had a baby or a pregnancy loss in the last 12 months. HbA1c is not used to diagnose diabetes in pregnancy or in the two months after it. John et al., Use of HbA1c in the diagnosis of diabetes mellitus in the UK, Diabetic Medicine 2012 (PMID 22957983)
- An infection or illness in the 6 weeks before. A severe illness at the time of the test can raise glucose for a while: UK guidance tests again once the person has recovered. John et al., Diabetic Medicine 2012 (PMID 22957983); WHO 2011 Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus, Annex 3
- Low iron or anaemia. Low iron can push HbA1c up without any change in blood sugar; anaemia from blood loss or broken-down red cells can pull it down. WHO 2011 Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus, Annex 1; John et al., Diabetic Medicine 2012 (PMID 22957983); Lab Tests Online UK: HbA1c test
- Gave blood, lost blood or had a transfusion. Heavy bleeding can make HbA1c read falsely low, and a recent blood transfusion makes it unreliable. Lab Tests Online UK: HbA1c test
Long-term conditions
- Diabetes. In known diabetes HbA1c is read against a target agreed with the person (NICE for type 2 diabetes: usually 48 or 53 mmol/mol), not against the thresholds used for diagnosis. NICE NG28 Type 2 diabetes in adults: management (2015, updated 2026), recs 1.5.5, 1.5.7
- Borderline blood sugar (prediabetes). After a result in the high-risk range NICE offers a blood test at least once a year. NICE PH38 Type 2 diabetes: prevention in people at high risk (2012, updated 2017), rec 1.6.5
- Diabetes during a pregnancy (gestational diabetes). After gestational diabetes NICE offers an HbA1c test every year. NICE NG3 Diabetes in pregnancy (2015, updated 2020), rec 1.6.14
- Kidney disease. In kidney failure HbA1c is not used to diagnose diabetes: anaemia and changed red cells make it read unpredictably. John et al., Diabetic Medicine 2012 (PMID 22957983); NICE NG28 (2015, updated 2026), rec 1.5.10
- Sickle cell or thalassaemia trait, or another unusual haemoglobin. HbA1c assumes ordinary adult haemoglobin: with sickle cell, thalassaemia or another variant the result depends on the laboratory method and may mislead. John et al., Diabetic Medicine 2012 (PMID 22957983); NICE NG28 (2015, updated 2026), rec 1.5.3; Lab Tests Online UK: HbA1c test
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
HbA1c
Read together with: Fasting glucose, Haemoglobin, Ferritin. NICE PH38 Type 2 diabetes: prevention in people at high risk (2012, updated 2017), rec 1.4.1; WHO 2011 Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus; John et al., Diabetic Medicine 2012 (PMID 22957983)
A GP usually repeats the test, looks at weight, blood pressure and cholesterol together with it, and refers people in the high-risk band to the NHS Diabetes Prevention Programme.
Fasting glucose
Read together with: HbA1c. NICE PH38 (2012, updated 2017), recs 1.4.1, 1.5.7; WHO 2011 Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus, section 2
A GP usually repeats it fasting or adds HbA1c, and looks at weight, blood pressure and family history alongside.
Questions to ask your GP
- Does this need repeating, and when?
- Am I eligible for the NHS Diabetes Prevention Programme?
- Which of my other results should be read together with this one?
- Was I fasting long enough for this to count?
- Should this be repeated with HbA1c?
- What would change if the second test agrees with this one?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What is an HbA1c test?
An HbA1c test measures the average level of sugar (glucose) in your blood over the last few months. The NHS uses it to diagnose type 2 diabetes and to monitor type 1 and type 2 diabetes. Results are usually given in mmol/mol.
NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026Do you need to fast for an HbA1c test?
No. The NHS says you do not need to do anything to prepare, and you can eat and drink as usual. A small amount of blood is usually taken from the inside of your elbow.
NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026What does an HbA1c of 42 or 48 mean?
On NHS guidance, 42 to 47 mmol/mol is the range where you may be at risk of getting diabetes, sometimes called prediabetes. 48 mmol/mol or above is the range where you may have diabetes, and if you have not been diagnosed, the test is done again to confirm. The NHS says these levels are a guide: if you already have diabetes, you may have agreed different levels to aim for with your care team.
NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026How can you lower your HbA1c?
The NHS lists lifestyle changes that bring blood sugar down: more exercise, losing weight and changes to what you eat. Some people also need medicine for diabetes, or a change to a treatment they already have, as their GP advises. Ask your GP which steps suit you.
NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026How often is HbA1c checked?
The NHS says people with diabetes are usually advised to have an HbA1c test every 3 to 6 months. Other conditions and some medicines can sometimes affect the result. Tell your GP or nurse about any you have.
NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026Got your result? Start with this question in your AI.
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Related
Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- NHS: Haemoglobin A1c (HbA1c) test, Page last reviewed 10 September 2026