Limitations of HbA1c tests

Caution is needed when interpreting HbA1c in the presence of conditions affecting red blood cell survival time or nonenzymatic glycation of haemoglobin. Red blood cell survival time is reduced in any type of haemolytic anaemia, and it can also be reduced in severe kidney impairment, severe liver disease and anaemia of chronic disease. The HbA1c in patients with any of these conditions often underestimates the true degree of hyperglycaemia.

Patients undergoing regular phlebotomy (eg for management of haemochromatosis, regular blood donors) may also have a falsely low HbA1c due to shortened red blood cell survival. Similarly, an HbA1c will not be an accurate reflection of the patient’s glucose if a patient had a recent blood transfusion.

Do not use HbA1c for diagnosis of diabetes or monitoring blood glucose concentrations for a woman within the first 3 months postpartum. HbA1c will be falsely low because of the lower blood glucose concentrations during pregnancy (due to haemodilution of pregnancy), the increased red blood cell turnover and/or shorter red cell lifespan in pregnancy, and peripartum blood loss.

Haemoglobinopathies can interfere with laboratory measurement of HbA1c level, although this may not be a problem with newer assays; check with the local laboratory. Iron deficiency can cause a falsely elevated HbA1c.

If there is a discrepancy between the HbA1c result and blood glucose concentration, consider the presence of a condition that may affect the HbA1c result. In this situation, use venous blood glucose concentrations instead of HbA1c to diagnose diabetes or to guide treatment.