HbA1c Explained
Why 2–3 Months?
Red blood cells live approximately 90–120 days. Glucose binds to haemoglobin in proportion to blood glucose concentration over this period. HbA1c therefore reflects average glucose over the lifespan of red cells — not just today's level.
HbA1c Diagnostic Thresholds
| HbA1c mmol/mol | % | Interpretation |
|---|---|---|
| <42 | <6% | Normal |
| 42–47 | 6.0–6.4% | Pre-diabetes (impaired glycaemia) — lifestyle intervention |
| ≥48 on two occasions | ≥6.5% | Type 2 diabetes diagnosed |
| ≥48 once with symptoms | ≥6.5% | Diabetes diagnosed (symptoms required for single-sample diagnosis |
Treatment Targets by Patient Type
| Patient Type | HbA1c Target |
|---|---|
| Most people with T2DM on lifestyle/metformin | <53 mmol/mol (<7%) |
| Elderly / at risk of hypoglycaemia | <59 mmol/mol (<7.5%) |
| Complex / end-stage / high hypoglycaemia risk | <64 mmol/mol (<8%) |
| Type 1 diabetes | <48–53 mmol/mol — individualised |
| Pregnancy (T1DM/T2DM preconception) | <48 mmol/mol — ideally <43 |
Conditions That Affect HbA1c AccuracyHaemoglobinopathies (sickle cell, thalassaemia), haemolytic anaemia, iron deficiency, and recent blood transfusion all affect red cell lifespan and can give falsely low or high HbA1c. Use fructosamine or continuous glucose monitoring in these patients.
Can I use HbA1c to diagnose type 1 diabetes?
HbA1c is used to diagnose type 2 diabetes but not type 1. Type 1 usually presents acutely with symptoms and high glucose — HbA1c may be falsely normal if the onset was rapid. Antibodies (GADA, IA-2A) confirm T1DM.
How quickly does HbA1c change?
It takes 2–3 months for HbA1c to fully reflect a change in glucose control. Significant improvements (e.g., starting medication or major lifestyle change) become apparent within 1–2 months but are fully reflected at 3 months.
Is HbA1c the same as fasting blood glucose?
No. Fasting glucose measures blood sugar at one point in time (morning fasting). HbA1c reflects average glucose over 2–3 months. Both are used for diagnosis; HbA1c is preferred as it doesn't require fasting.
What makes HbA1c go up?
Poor dietary control, lack of exercise, weight gain, illness (steroids, infections), and medication non-adherence. Understanding the specific driver helps target the intervention.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.