HbA1c vs Fasting Glucose - what's the difference?
Both measure blood sugar, but differently: HbA1c is a ~3-month average, fasting glucose is a snapshot. Here's when each matters.
HbA1c
Glycated hemoglobin - the share of hemoglobin "sugar-coated" over the last ~2–3 months. Reflects your average sugar, independent of your last meal.
Fasting glucose
Your blood-sugar level at a single moment, usually after 8+ hours fasting. Changes quickly with food, stress and exercise.
HbA1c vs Fasting glucose
| Hemoglobin A1c (HbA1c) | Fasting Blood Glucose | |
|---|---|---|
| Unit | % | mg/dL |
| Typical adult range | 4–5.6 % | 70–99 mg/dL |
| Common reasons for a high value | 5.7–6.4%; ≥6.5% | 100–125 mg/dL; ≥126 mg/dL on two occasions |
| Common reasons for a low value | Recent significant blood loss; Hemolytic anaemia | Hypoglycaemia; Insulin overdose / sulfonylurea |
Key differences
- HbA1c averages ~3 months; fasting glucose is a point-in-time snapshot.
- HbA1c needs no fasting; fasting glucose does.
- Glucose is sensitive to your last meal and stress; HbA1c isn't.
- A diabetes diagnosis is often made using both together.
How does HbA1c translate into average glucose?
In the ADAG study, HbA1c was converted into an estimated average glucose: average glucose (mg/dL) = 28.7 × HbA1c (%) − 46.7. An HbA1c of 7% corresponds to an average of about 154 mg/dL (8.6 mmol/L). An individual's own average can differ from this line.
Bottom line
It's not either/or: HbA1c gives the long-term picture, fasting glucose the current one. Your doctor usually looks at both.
Frequently asked questions
Which is more accurate for diabetes?
Both are valid and complementary. HbA1c is more convenient (no fasting, more stable), while fasting glucose catches acute changes.
Can HbA1c be normal while glucose is high?
Yes, after a recent sugar spike - HbA1c hasn't "caught up" yet. That's why they're read together.
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