HbA1c results can mislead if red blood cell lifespan varies

HbA1c, a common test for average blood sugar over three months, can give misleading results when red blood cells do not live their normal 120-day lifespan. The test measures glucose stuck to haemoglobin inside red blood cells. If cells are replaced faster, they pick up less glucose and HbA1c reads falsely low. If they survive longer, they collect more glucose and HbA1c reads falsely high. Two people with the same blood sugar can therefore get different HbA1c numbers.

Conditions that lengthen red blood cell survival include iron deficiency anaemia and vitamin B12 deficiency, which can push HbA1c up even when sugar control is reasonable. Conditions that shorten survival include haemolytic anaemia, chronic kidney disease, recent blood loss, pregnancy and recent blood transfusions, which can make HbA1c appear falsely low. When daily home glucose readings disagree with HbA1c results over several months, the test may not reflect true glucose levels.

Doctors can use fasting and post-meal blood glucose readings, home glucose logs, fructosamine (which covers two to three weeks and does not depend on red blood cell lifespan) or continuous glucose monitoring instead. Treating any underlying anaemia or deficiency may make future HbA1c results more reliable.