Glycosylated haemoglobin (HbA1c) and accuracy as a measure of glycaemia

• the value of HbA1c is thought, by many clinicians, to provide an accurate measure of of glycaemic control in diabetes
  • however there exists large intra-individual differences exist in the relationship between population mean plasma glucose levels and HbA1c
    • the DCCT (1) revealed that for patients with a mean plasma glucose of 10 mmol/l, HbA1c values ranged between 6% and 11%
      • therefore aiming for the same HbA1c in all patients therefore will mean that the target will be unrealistically low in some (with potential risk of hypoglycaemia) and unrealistically high in others, providing false reassurance (2)
      • HbA1c measurements rely on a predictable effect of glucose concentration on hemoglobin (Hb) over a normal red blood cell (RBC) life span - however any condition that alters RBC survival may invalidate HbA1c as an accurate measure of glycaemic control
        • for example hemolytic anaemias results in decreased RBC survival and hence reduced HbA1c;
        • iron deficiency anaemia results in a modest reduction in RBC survival
        • samples containing variant Hb cause erroneous results in measurement of HbA1c
      • note that a 95% around an HbA1c result of 7% would be 6.6% to 7.4% if there was a total coefficient of variation of 2.5%
    • therefore the clinician must consider these potential sources of variation when interpreting an HbA1c result

For more details regarding variation in HbA1c with specific conditions then see linked items.

Reference:

1. Rohlfing, C.L. et al. Defining the relationship between plasma glucose and HbA(1c): analysis of glucose profiles and HbA(1c) in the Diabetes Control and Complications Trial. Diabetes Care 2002;25(2): 275-278.
2. Kilpatrick, E.S. et al.Biological variation of glycated hemoglobin. Implications for diabetes screening and monitoring. Diabetes Care 1998;21(2): 261-264