What it is
Estimated average glucose (eAG) translates an HbA1c into the same mg/dL (or mmol/L) units a patient sees on a home glucose meter, representing the average glucose over the preceding ~2–3 months. It comes from the ADAG study (A1c-Derived Average Glucose), which regressed frequent CGM/SMBG glucose against measured A1c.
Formula
$$ eAG,(mg/dL) = 28.7 \times A1c,(%) - 46.7 $$
$$ eAG,(mmol/L) = \frac{eAG,(mg/dL)}{18} $$
The ADAG regression had a tight fit (R² = 0.84) and did not differ significantly by age, sex, diabetes type, race/ethnicity, or smoking.
When to use
Whenever you want to make an A1c tangible for a patient or to sanity-check an A1c against their meter/CGM averages. A large mismatch between the eAG and the actually-measured average glucose is itself a clinical signal worth chasing.
Worked example
HbA1c 8.0%:
- eAG = 28.7 x 8.0 - 46.7 = 229.6 - 46.7 = 182.9 ≈ 183 mg/dL
- In SI units = 182.9 / 18 ≈ 10.2 mmol/L
So an A1c of 8% corresponds to an average glucose around 183 mg/dL (10.2 mmol/L). (For reference, an A1c of 7% → ~154 mg/dL.)
Pitfalls
- A1c–eAG discordance is informative, not noise. When the calculated eAG is far off the patient’s CGM/meter average, suspect anything that alters the relationship between glucose and glycation: haemoglobinopathies, altered RBC lifespan (haemolysis, recent blood loss → falsely low A1c), iron-deficiency anaemia (can raise A1c), pregnancy, and recent transfusion.
- It is an estimate with a confidence band. The ADAG point estimate hides a meaningful 95% interval — two patients with the same A1c can have genuinely different true averages. Don’t treat eAG as the patient’s literal mean.
- eAG is a long-run average weighted toward the most recent 4–6 weeks — it tells you nothing about glycaemic variability or hypoglycaemia, which CGM time-in-range metrics capture and A1c cannot.
- Assay matters — use an NGSP-aligned A1c; point-of-care and lab methods are not always interchangeable.
Run it: HbA1c to Estimated Average Glucose (eAG)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.