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Glycosylated haemoglobin (Hb A1) measurement is the most widely used measure of longterm glycaemic control in diabetes.

Glycosylated haemoglobin is produced by the non-enzymatic glycosylation of haemoglobin at a rate proportional to the prevailing glucose concentration. The level of Hb A1 depends upon:

  • red cell lifespan
  • prevailing blood glucose concentration

Providing red cell lifespan is normal, Hb A1 measures mean blood glucose concentration over the preceding 60 days - i.e. half-life of red cell.

Some assays measure total glycosylated haemoglobin whilst others measure Hb A1c produced by glycosylation of the N-terminal valine of the B-chain of haemoglobin.


  • the HbA1c results will be reported exclusively as mmol/mol of haemoglobin without glucose attached, rather than a percentage as previously, from June 1st 2011
  • the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) has initiated the change:
    • the equivalent of the current DCCT HbA1c targets of 6.5% and 7.5% are 48 mmol/mol and 59 mmol/mol in the new units, with the nondiabetic reference range of 4.0% to 6.0% being 20 mmol/mol to 42 mmol/mol
  • DCCT- HbA1c (%) IFCC-HbA1c (mmol/mol)
    6.0 42
    6.5 48
    7.0 53
    7.5 59
    8.0 64
    9.0 75

Last reviewed 05/2021