Endocrinology
HbA1c to Average Glucose
Estimated average glucose from HbA1c.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
This conversion translates a glycated haemoglobin (HbA1c) result into an estimated average glucose (eAG), expressing long-term glycaemic control as an everyday glucose value. HbA1c reflects average blood glucose over roughly the preceding two to three months, the lifespan of red cells, but the percentage is unfamiliar to many patients. The ADAG study derived a linear relationship between HbA1c and continuously monitored mean glucose. In mmol/L, eAG is approximately 1.59 × HbA1c(%) − 2.59, allowing HbA1c to be discussed in the same units as a home glucose meter.
Interpreting the result
The result is an average, not a target or a single reading: a given eAG smooths over the highs and lows a patient actually experiences. For orientation, an HbA1c of about 7% corresponds to an eAG of roughly 8.6 mmol/L, and each 1% rise in HbA1c adds roughly 1.6 mmol/L to the average glucose. The conversion aids patient understanding and goal-setting but does not replace HbA1c for diagnosis or for assessing control. It is the relationship, not a precise individual prediction, that matters.
Worked example
A patient has an HbA1c of 8%. eAG ≈ 1.59 × 8 − 2.59 = 12.72 − 2.59 ≈ 10.1 mmol/L. This means their average glucose over recent weeks has been around 10 mmol/L, a figure many patients find easier to relate to than the percentage.
Critical actions
eAG (mmol/L) ≈ 1.59 × HbA1c(%) − 2.59. This is an estimate of average glucose, not a target. Enter HbA1c as a percentage (DCCT/NGSP).
Pearls / pitfalls
- eAG is an average — it can mask wide glucose swings, so it does not replace self-monitoring or continuous glucose data for day-to-day decisions.
- Enter HbA1c as a DCCT/NGSP percentage; if you have an IFCC value in mmol/mol, convert it to a percentage first.
- Conditions that alter red-cell turnover — anaemia, haemoglobinopathies, recent transfusion, pregnancy — distort HbA1c and therefore the eAG.
- The eAG is an estimate around a population regression line; an individual's true average may differ, so present it as approximate.
Evidence & validation
Based on the international A1c-Derived Average Glucose (ADAG) study (Nathan et al., 2008), which correlated HbA1c with continuous and frequent glucose monitoring; the relationship is endorsed by the ADA and used widely in diabetes education.
Frequently asked questions
Is the estimated average glucose a target?
No — it simply restates the HbA1c as an average glucose to aid understanding. Targets are set individually and are usually still expressed as an HbA1c.
Does it replace finger-prick or sensor glucose?
No. eAG is a single averaged figure and cannot show the daily highs and lows that guide treatment adjustments. Self-monitoring or continuous glucose monitoring remains necessary for that.
My HbA1c is in mmol/mol — can I still use this?
Yes, but convert the IFCC mmol/mol value to a DCCT/NGSP percentage first, since the formula expects a percentage. Many laboratories and apps report both.
Why might eAG be misleading in some patients?
Anything that changes red-cell lifespan — such as anaemia, haemoglobinopathies, recent transfusion or pregnancy — distorts HbA1c and therefore the estimated average glucose. In these situations interpret with caution.
How accurate is the conversion?
It reflects a population-level relationship, so an individual's true average glucose may differ somewhat from the estimate. It is best presented as an approximate guide rather than a precise value.
References
- Nathan DM, Kuenen J, Borg R, et al. Translating the A1C assay into estimated average glucose values (ADAG study). Diabetes Care. 2008;31(8):1473–1478.
- American Diabetes Association. Standards of Care in Diabetes — Glycemic Targets. Diabetes Care. 2024;47(Suppl 1).
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