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What are HbA1c and eAG?
HbA1c (glycated hemoglobin) shows the proportion of glucose bound to the hemoglobin in your red blood cells. Because red blood cells live about 3 months, HbA1c reflects your average blood sugar over the past 2–3 months — unlike a single fasting measurement, it gives the long-term picture. eAG (estimated average glucose) then translates that percentage into the mg/dL (or mmol/L) unit you see on a daily blood sugar reading. This way you can see concretely what “average” means for a HbA1c of 6.5%.
How is the A1c – eAG conversion done?
The conversion is based on the linear equation of the ADAG (A1c-Derived Average Glucose) study. The HbA1c percentage is converted into average glucose as follows:
eAG (mmol/L) = 1.59 × HbA1c − 2.59
IFCC (mmol/mol) = (HbA1c % − 2.15) × 10.929
Example: HbA1c = 7.0%
28.7 × 7 − 46.7 = 154.2 → ≈ 154 mg/dL average sugar
(mmol/L: 1.59 × 7 − 2.59 = 8.54; IFCC: (7 − 2.15) × 10.929 ≈ 53 mmol/mol). According to the ADA, 6.5% and above is in the diabetes range.
Method: The eAG equations come from the ADAG study (Nathan DM et al., Diabetes Care, 2008). The IFCC ↔ NGSP conversion uses the IFCC-NGSP master equation. Status thresholds: American Diabetes Association (ADA) — normal <5.7%, prediabetes 5.7–6.4%, diabetes ≥6.5%.
What you need to know about HbA1c
- It is an average, not a snapshot: HbA1c does not show daily ups and downs; it gives the overall trend. Day-to-day management needs fingerstick/CGM readings.
- Some conditions skew the value: Anemia, pregnancy, chronic kidney disease, recent blood loss and certain hemoglobin variants can make HbA1c misleading.
- It does not diagnose on its own: eAG is an estimate; diagnosis and treatment decisions rest on your physician's measurements and assessment.
- The target varies by person: For most adults the target is generally below 7%, but it is individualized based on age and coexisting conditions.
Frequently Asked Questions
According to the ADAG equation, a HbA1c of 7.0% corresponds to an average blood sugar of about 154 mg/dL (8.5 mmol/L). This is the average over the past 2–3 months, not a single snapshot measurement.
eAG represents the average of the entire day (including sleep), while a fingerstick shows the value at that moment. Fasting, post-meal and overnight values differ, so it is normal for a spot reading to be higher or lower than the eAG.
IFCC is the international standard unit for HbA1c and is expressed in mmol/mol. It converts from the widely used percentage (NGSP) unit with this equation: IFCC = (percentage − 2.15) × 10.929. For example, 7% ≈ 53 mmol/mol.
According to the ADA, HbA1c: below 5.7% is normal, 5.7–6.4% is prediabetes, and 6.5% and above is diabetes. Diagnosis is usually confirmed by a physician with two separate measurements or additional tests.
The eAG formula is derived from the ADAG study, which compared HbA1c with continuous glucose measurements, and it shows a strong correlation across the population. However, individual differences can occur; anemia, pregnancy or certain hemoglobin variants can skew the value.
As a general rule, twice a year is recommended for people who have reached their target and are stable, and every 3 months for those with a treatment change or who have not reached their target. Follow your physician's plan for the exact frequency.