Comparison · Medical test vs Medical test
HbA1c vs blood glucose
Blood glucose is a snapshot of one moment; HbA1c is a three-month average. Both diagnose diabetes, but they answer different questions and each fails in different circumstances.
| Aspect | HbA1c | Blood glucose |
|---|---|---|
| What it measures | The fraction of haemoglobin with glucose attached, reflecting average glucose over 2-3 months | Glucose concentration in blood at the time of sampling |
| Preparation | None; no fasting | Fasting for 8-12 hours for a fasting test; timed after a glucose drink for a tolerance test |
| Diagnostic threshold (UK) | 48 mmol/mol (6.5%) or above, repeated unless symptomatic | Fasting 7.0 mmol/L or above, or 11.1 mmol/L two hours after a 75 g glucose load |
| Day-to-day variation | Very little | Large: meals, exercise, stress and illness move it within hours |
| Not reliable in | Anaemia, haemolysis, transfusion, pregnancy, haemoglobin variants, kidney failure, suspected type 1 diabetes | Delayed samples without fluoride, meter inaccuracy, acute stress |
| Monitoring role | Every 3-6 months for treatment targets | Daily self-monitoring on insulin; continuous glucose monitoring |
Where HbA1c fits
Diagnosing and monitoring type 2 diabetes without fasting, and setting treatment targets
Where Blood glucose fits
Suspected type 1 diabetes, hypoglycaemia, pregnancy, acute illness, and any situation where red-cell lifespan is abnormal
Educational content. A comparison explains what each test, marker or medicine is for and how they differ. It is educational and does not say which one a person should have; that decision belongs to the clinician who knows the person and the question.