Convert HbA1c: percent (%) to mmol/mol and back

Two units for the same value

HbA1c (long-term blood sugar) was long reported in percent (%), following the US NGSP standard. Since international harmonisation (IFCC) many countries report it in mmol/mol. Reports often show both values.

The formula

IFCC (mmol/mol) = (NGSP in % − 2.15) × 10.929. In reverse: NGSP (%) = IFCC × 0.09148 + 2.15. There is no single factor because the two scales are offset against each other.

Worked example

6.5 % gives (6.5 − 2.15) × 10.929 = 47.5 mmol/mol, so about 48 mmol/mol. In reverse, 53 mmol/mol equals about 53 × 0.09148 + 2.15 = 7.0 %.

Common mistake

Using a plain factor (for example times 10) gives wrong values. The offset of 2.15 has to be included. HbA1c is also a proportion rather than a concentration, so conversion via molar mass does not apply.

What the number means

Whether a value is normal or needs treatment is for your doctor to decide. Targets depend on age, other conditions and treatment. MedConvert only shows orientation ranges with a source.

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• HbA1c
• Glucose

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