Key Takeaways
- HbA1c is the primary blood test used to diagnose and monitor type 2 diabetes in the UK.
- A result of 48 mmol/mol (6.5%) or above indicates diabetes; 42–47 mmol/mol indicates pre-diabetes.
- Fasting blood glucose and the oral glucose tolerance test (OGTT) are also used, particularly in pregnancy.
- Regular monitoring can detect diabetes years before symptoms appear, when intervention is most effective.
Diabetes in the UK: The Scale of the Problem
Diabetes affects approximately 5 million people in the UK, with type 2 diabetes accounting for around 90% of cases. Diabetes UK estimates that a further 850,000 people have type 2 diabetes but have not yet been diagnosed, and 13.6 million people are at increased risk of developing the condition.
Blood tests are the only definitive way to diagnose diabetes. Symptoms such as increased thirst, frequent urination, fatigue, and blurred vision often develop gradually and can be dismissed as unrelated. Many people are diagnosed incidentally when blood tests are performed for another reason entirely.
HbA1c (Glycated Haemoglobin)
The HbA1c test is the gold standard for diagnosing and monitoring type 2 diabetes in the UK. It measures the percentage of haemoglobin in your red blood cells that has glucose attached to it. Because red blood cells live for approximately 120 days, HbA1c reflects your average blood sugar level over the previous 2 to 3 months.
UK Diagnostic Thresholds
| HbA1c (mmol/mol) | HbA1c (%) | Interpretation |
|---|---|---|
| Below 42 | Below 6.0% | Normal |
| 42–47 | 6.0–6.4% | Pre-diabetes (high risk) |
| 48 or above | 6.5% or above | Diabetes |
Advantages of HbA1c
- No fasting required — the test can be done at any time of day, regardless of when you last ate.
- Reflects long-term blood sugar control, not just a single point in time.
- Less affected by day-to-day variation, stress, or illness.
- A single sample is sufficient for diagnosis (though NICE recommends a repeat test to confirm).
Limitations of HbA1c
- Not reliable in conditions that affect red blood cell turnover (sickle cell disease, thalassaemia, recent blood transfusion, iron deficiency anaemia).
- Not recommended for diagnosing type 1 diabetes, where symptoms usually develop rapidly.
- Not used in pregnancy — the oral glucose tolerance test is preferred for gestational diabetes.
Fasting Blood Glucose
A fasting blood glucose test measures the concentration of glucose in your blood after an overnight fast of at least 8 hours. It provides a snapshot of your blood sugar level at a single point in time. For guidance on fasting, see our article on eating before blood tests.
| Fasting Glucose (mmol/L) | Interpretation |
|---|---|
| Below 5.5 | Normal |
| 5.5–6.9 | Impaired fasting glucose (pre-diabetes) |
| 7.0 or above | Diabetes (if confirmed on a second test) |
Oral Glucose Tolerance Test (OGTT)
The OGTT is a two-stage test. After an overnight fast, a baseline blood glucose is taken. You then drink a solution containing 75g of glucose, and a second blood sample is taken 2 hours later. The test measures how efficiently your body processes sugar.
The OGTT is the standard test for diagnosing gestational diabetes (diabetes in pregnancy) and is recommended by NICE for all pregnant women with risk factors. It is also used when HbA1c results are inconclusive or unreliable.
| 2-Hour Glucose (mmol/L) | Interpretation |
|---|---|
| Below 7.8 | Normal |
| 7.8–11.0 | Impaired glucose tolerance (pre-diabetes) |
| 11.1 or above | Diabetes |
Random Blood Glucose
A random blood glucose test can be taken at any time, without fasting. It is typically used when a patient presents with classic diabetes symptoms (extreme thirst, frequent urination, unexplained weight loss). A random glucose of 11.1 mmol/L or above, combined with symptoms, is sufficient for a diabetes diagnosis without further testing.
Pre-Diabetes: The Window of Opportunity
Pre-diabetes means your blood sugar is higher than normal but not yet high enough for a diabetes diagnosis. It is significant because it represents a window of opportunity: with lifestyle changes (diet, exercise, weight management), many people with pre-diabetes can reduce their HbA1c back to normal levels and avoid progressing to type 2 diabetes.
The NHS Diabetes Prevention Programme is available free of charge to people in England who have been identified as having pre-diabetes. It provides coaching on healthy eating, physical activity, and weight management over a 9-month period.
Monitoring Existing Diabetes
If you have been diagnosed with type 2 diabetes, your GP will typically check your HbA1c every 3 to 6 months. The target for most people with diabetes is an HbA1c below 48 mmol/mol (6.5%), though this may be adjusted depending on your medication and individual circumstances.
Regular monitoring also includes kidney function (annual urine albumin-to-creatinine ratio and eGFR), lipid profile, blood pressure, and eye and foot examinations. These are part of the annual diabetes review that all people with diabetes should receive from their GP or diabetes clinic.
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