How this calculator works
The ADAG study relates laboratory HbA1c to glucose averaged over roughly three months. This page preserves its separately rounded mg/dL and mmol/L equations. NGSP percent and IFCC mmol/mol describe HbA1c on different reporting scales; their conversion is a separate standardisation relationship. The reverse glucose calculation is a statistical estimate of HbA1c, not a laboratory test. The supported 4-12% NGSP range avoids extrapolation beyond the ADAG table. Altered red-cell survival, anaemia and some haemoglobin variants can make the glucose relationship unsuitable. The result is an estimate for information only and is not medical advice.
Formula
Let A be NGSP HbA1c in percent. eAG(mg/dL) = 28.7A - 46.7; eAG(mmol/L) = 1.59A - 2.59. Reverse: A = (eAGmg/dL + 46.7)/28.7 or (eAGmmol/L + 2.59)/1.59. IFCC(mmol/mol) = 10.93A - 23.50; its algebraic inverse is A = (IFCC + 23.50)/10.93.
Worked example
HbA1c of 7% NGSP corresponds to 53.01 mmol/mol IFCC, estimated average glucose of 154.2 mg/dL and 8.54 mmol/L. Starting with 8.54 mmol/L average glucose gives (8.54 + 2.59)/1.59 = 7% NGSP. At 6% NGSP, the corresponding figures are 42.08 mmol/mol, 125.5 mg/dL and 6.95 mmol/L.
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Frequently asked questions
Can a single glucose reading estimate HbA1c?
No. Use a representative average over time. A single fasting or after-meal reading is not the average used in ADAG.
Why do the two glucose equations differ slightly from a unit conversion?
ADAG reports separately rounded regression coefficients. Each equation is used as published instead of replacing one with a molecular-weight conversion.
Why use an algebraic inverse for the IFCC conversion?
This page reverses the displayed IFCC = 10.93 x NGSP - 23.50 relationship exactly. The independently rounded reverse coefficients also published by NGSP can differ slightly in the last decimals.
Does an estimated HbA1c replace clinical testing?
No. It does not diagnose diabetes, choose a treatment target or account for conditions that change red-cell turnover. Interpretation belongs with an appropriate clinical assessment.