Glycated haemoglobin levels (HbA1c) has become one of the key targets in diabetes care, understood by patients and their health care professional. However, clinicians working in diabetes cannot fail to have noticed that their local chemical pathology departments have also been measuring HbA1c using a less familiar scale than the well known percentage scale, mmol/mol. This dual reporting has been going on since June 2009 but from June 2011 (Scotland Wales and Northern Ireland) and October 2011 (England), only the mmol/mol value will be recorded. But, why the change?
To explain this, a brief history of HbA1c is required. HbA1c was first used in routine clinical laboratories for diabetes monitoring around 1977. At the time all methods had either no calibrators, or was performed using material with assayed values derived from individual manufacturers’ assays. Over the next five to fifteen years, inter-laboratory variability improved markedly. The use of a precise HPLC method as the “standard method” in the Diabetes Control and Complications Trial (DCCT) led to significant further improvement. Since, all HbA1c values including those from the large UKPDS study have been standardised to the DCCT allowing international interpretation of results and subsequent clinical targets. However, the method used for measuring HbA1c in the DCCT has increasingly been found to have a variety of interferences. HbA1c results from newer assay methods, although now free from interferences have always been adjusted to give results aligned to the old DCCT method, hence the reference to DCCT on the biochemistry reports.
As a result, the International Federation of Clinical Chemistry (IFCC) have produced an international standard assay free from interference but measured in mmol/mol in order to avoid confusion with the old scale.
Conversion tables are widely available both in hard copy and on various websites e.g. Diabetes UK. As an example, an HbA1c of 7% is equivalent to 53mmol/mol whilst 10% equates to 86mmol/mol. Various formulae have also been devised for those with an aptitude for mental maths.
Such a fundamental change to a key clinical indicator will take some time to be accepted despite the prolonged period of dual reporting but is likely to have the added educational benefit of patients confusing their HbA1c and their home monitoring readings. However, in the longer term it is likely that the transition will go smoothly – after all, most people do not realise that petrol now costs £6.50 per gallon (or is that 130 shillings!)
Dr. Mark Freeman
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