HbA1c (or A1c) is a single number that summarizes the past 2–3 months of blood sugar control.
What it measures
Glucose in your bloodstream binds to hemoglobin in your red blood cells. The percentage that's "sugared" gives you HbA1c. Since red cells live 3 months, you get a 90-day average.
Reading the number
| HbA1c | Avg glucose (mg/dL) | Avg glucose (mmol/L) |
|---|---|---|
| 5.0% | 97 | 5.4 |
| 5.7% | 117 | 6.5 |
| 6.0% | 126 | 7.0 |
| 6.5% | 140 | 7.8 |
| 7.0% | 154 | 8.6 |
| 8.0% | 183 | 10.2 |
| 9.0% | 212 | 11.8 |
| 10.0% | 240 | 13.4 |
Targets
- **General population**: under 5.7%
- **ADA target for diabetes**: under 7.0%
- **Tight control (younger, no hypo risk)**: under 6.5%
- **Older / high hypo risk**: 7.5–8.0% acceptable
What HbA1c misses
A 7.0% can mean steady 154 mg/dL — or wild swings averaging 154. Two people with the same A1c can have very different cardiovascular risks. CGM data fills in this picture by showing time in range (% of day spent between 70–180 mg/dL).
When testing is unreliable
- After a recent blood transfusion
- With anemia or other red-cell disorders
- During pregnancy
- With certain hemoglobin variants (more common in some ethnicities)
In these cases, fructosamine (2–3 week average) is more reliable.