In more detail
Glucose in the blood attaches to hemoglobin inside red blood cells. Those cells live about three months, so the share of glycated hemoglobin reflects average blood sugar over that period rather than on the day of the test, and no fasting is needed. Results are reported in per cent (NGSP units) or in mmol/mol (IFCC units, (A1C − 2.15) × 10.929). An estimated average glucose (eAG) follows from 28.7 × A1C − 46.7 in mg/dL; divide by 18 for mmol/L.
Why it matters
Widely used diagnostic cut-offs are below 5.7% normal, 5.7 to 6.4% prediabetes and 6.5% or more diabetes, normally confirmed by a repeat test. Anything that changes the lifespan of red cells, such as anaemia, recent blood loss or transfusion, pregnancy, kidney disease or some hemoglobin variants, can make the result read falsely high or low. Mixing up per cent and mmol/mol (7% versus 53) is a common source of confusion. Personal targets and diagnoses are for a clinician to set.
Example
A1C eAG (mg/dL) eAG (mmol/L) IFCC (mmol/mol)
5.7% 116.9 6.5 38.8
6.5% 139.8 7.8 47.5
7.0% 154.2 8.6 53.0
8.0% 182.9 10.2 63.9