Intro
HbA1c (also called A1C) reflects your average blood glucose over approximately 3 months by measuring how much glucose has attached to haemoglobin in red blood cells. It is used to diagnose diabetes and prediabetes and to track long-term glucose control in people already diagnosed.
Key Points
- HbA1c is an average of the last ~8–12 weeks, not a spot glucose reading
- Diabetes diagnosis: HbA1c ≥6.5% (48 mmol/mol); prediabetes 5.7–6.4% (ADA). Use standardised assays and confirm with repeat testing
- Typical target for many adults with diabetes: HbA1c <7% — individualise based on age, hypoglycaemia risk, and comorbidity
- Pair HbA1c with CGM Time-in-Range (TIR) for day-to-day quality and hypoglycaemia safety — aim ≥70% TIR (70–180 mg/dL / 3.9–10.0 mmol/L), with <4% below 70 mg/dL
- HbA1c can be misleading with anaemia, haemoglobin variants, kidney disease, pregnancy, or recent transfusion
What HbA1c Measures
Glucose slowly attaches to haemoglobin inside red blood cells over their ~3-month lifespan. The percentage that is glycated — called HbA1c — tracks your mean glucose exposure over that period. Higher average glucose produces a higher HbA1c.
This is fundamentally different from a spot blood glucose reading, which captures only a single moment. HbA1c is stable across the day and is not affected by what you ate on the morning of the test.
How It’s Used in Practice
Diagnosis
| HbA1c | Interpretation |
|---|---|
| Below 5.7% (39 mmol/mol) | Normal (ADA) |
| 5.7–6.4% (39–47 mmol/mol) | Prediabetes (ADA) |
| 6.5% (48 mmol/mol) or above | Diabetes |
Always confirm with a repeat test or corroborating glucose measurement unless classic symptoms and markedly elevated glucose are present. Use a lab method aligned to IFCC/NGSP standards; when results conflict with the clinical picture, repeat or use an alternative test.
Monitoring and targets
- Test every 6 months if stable and at goal; every 3 months if therapy changed or not at goal
- Many adults with diabetes target HbA1c <7% — but targets are individualised for older adults, children, pregnancy, and people at high hypoglycaemia risk
HbA1c ↔ Estimated Average Glucose (eAG)
The ADAG formula converts HbA1c to an estimated average glucose:
- eAG (mg/dL) = 28.7 × A1C − 46.7
- eAG (mmol/L) = (28.7 × A1C − 46.7) ÷ 18
| HbA1c (%) | eAG (mg/dL) | eAG (mmol/L) |
|---|---|---|
| 5 | 97 | 5.4 |
| 6 | 125 | 7.0 |
| 7 | 154 | 8.6 |
| 8 | 183 | 10.2 |
| 9 | 212 | 11.8 |
| 10 | 240 | 13.4 |
| 11 | 269 | 14.9 |
| 12 | 298 | 16.5 |
eAG is an estimate. Day-to-day swings, hypoglycaemia episodes, and overnight glucose patterns may not be visible from HbA1c alone.
HbA1c + Time-in-Range: Better Together
HbA1c shows the average; TIR from a CGM shows how often you’re in the target band and how much time is spent dangerously low or high.
General adult TIR targets: ≥70% in range (70–180 mg/dL / 3.9–10.0 mmol/L), with <4% below 70 mg/dL and <1% below 54 mg/dL. These targets are individualised for older adults, frailty, and pregnancy.
Limitations and Misunderstandings
Consider alternative markers (e.g. fructosamine or glycated albumin) or rely more on CGM metrics when any of these apply:
- Anaemia: iron deficiency can raise HbA1c artificially; haemolysis can lower it
- Haemoglobin variants (e.g. HbS, HbC): some assays are affected; specialist interpretation may be required
- Recent transfusion or erythropoietin therapy: alters red blood cell lifespan and affects HbA1c accuracy
- Dialysis or CKD: HbA1c can underestimate true glucose exposure
- Pregnancy: not used to diagnose gestational diabetes; red blood cell turnover is altered, making results unreliable
FAQ
What does HbA1c measure? HbA1c measures the percentage of haemoglobin in your red blood cells that has glucose attached to it. Because red blood cells live about 3 months, it reflects your average blood glucose over that period — not a single moment in time.
What HbA1c level means I have diabetes? A confirmed HbA1c of 6.5% (48 mmol/mol) or above is used to diagnose diabetes. Prediabetes is 5.7–6.4% (ADA). Results should be confirmed with a repeat test or corroborating glucose measurement.
What is a good HbA1c target? Many non-pregnant adults with diabetes aim for below 7% (53 mmol/mol). The right target depends on age, hypoglycaemia risk, and other health conditions. Targets are always individualised — discuss with your clinician.
How often should I have my HbA1c tested? Every 6 months if stable and at goal; every 3 months if treatment has recently changed or levels are not at target (per ADA Standards of Care).
Can HbA1c give inaccurate results? Yes. Conditions that affect red blood cell lifespan — iron-deficiency anaemia, haemolysis, recent transfusion, CKD, or certain haemoglobin variants (e.g. HbS, HbC) — can make HbA1c misleadingly high or low.
Is HbA1c the same as my “average” on CGM? They usually correlate, but CGM averages and HbA1c can diverge due to sensor use time, day-night glucose variability, anaemia, or individual differences in RBC lifespan. Use both HbA1c and TIR for the full picture.
What is eAG (estimated average glucose)? eAG converts your HbA1c to an approximate average glucose in mg/dL or mmol/L using the ADAG formula. It is an estimate — hypoglycaemia episodes and overnight glucose patterns may not be reflected in the value.
Can a single high blood glucose reading raise my HbA1c? Not significantly. HbA1c reflects the average across the whole 3-month period, changing gradually as overall control improves or worsens over weeks.
My HbA1c looks “too good” for my meter readings — why? Check for factors that lower HbA1c artificially, such as haemolysis, recent blood loss or transfusion, or a haemoglobin variant. Discuss with your clinician.
Further Reading
- ADA — Understanding A1C (patient information)
- ADA — Standards of Medical Care in Diabetes (diagnosis and monitoring thresholds)
- WHO — Use of HbA1c for Diagnosis of Diabetes Mellitus
- International Consensus on Time-in-Range targets
Related Guides
- Type 1 vs Type 2 Diabetes — Key Differences
- Newly Diagnosed with Type 1 Diabetes — First Steps
- Blood Glucose Testing — How and When to Check
- Diabetes Emergencies — Hypos, Highs, and Ketones
- Prediabetes: Early Warning Signs and Prevention
Educational only; not a substitute for professional medical advice.