Change to reporting of results for Hemoglobin A1c (HbA1c)

(Aotea News, August 2009)

Dr Michael Crooke
Chemical Pathologist

From August 3rd 2009, the units reported for HbA1c by laboratories in the Wellington region will change. Other laboratories in New Zealand will make the same change on this date. The new mode of reporting has already been implemented in the UK and information is available at this web page.

The change is being made in response to an international consensus statement, published in 2007, which has been endorsed in a position statement from the New Zealand Society for the Study of Diabetes, dated March 2009. Read more at this page. 

Currently, HbA1c is reported in % and the assays are standardised against the methods used in major clinical trials, especially the Diabetes Control and Complications Trial [DCCT]. These trials showed that risk of microvascular complications of diabetes is related to levels of HbA1c and the data has been used to set the clinical targets we are familiar with, eg HbA1c of 7% in a DCCT aligned assay represents a reasonable target for good control.

The problem is that the method used in the DCCT is not a true reference method and is not suitable for long term standardisation of HbA1c, as it does not measure HbA1c with sufficient specificity and is subject to drift, causing loss of accuracy. For those interested in the world of metrology, a reference method is one which is calibrated with pure standards to measure an exactly defined substance with known and very small uncertainty. Reference methods are completely unsuitable for routine use and are maintained at only a very few sites in the world. They are used to assign values to calibration materials used in the routine assays used in clinical practice, thus ensuring traceability of results back to the reference method and the ability to be able to validly compare results, worldwide.

A new, internationally accepted reference method has been developed for HbA1c and this is the basis for the change in reporting. This reference method is maintained by the International Federation of Clinical Chemistry [IFCC]. All routine methods for HbA1c will be calibrated against this method. The new reference method reports in correct SI units rather than % and these units are mmol/mol.

The new units are numerically very different from the old units, eg 7% derived from a DCCT aligned assay will be 53mmol/mol in new units. The table gives conversions for other levels. This level of 53mmol/l has the same clinical significance as 7% and will now be the target for good control.

To allow everyone to become familiar with the new units we will be reporting both results for a period of 2 years. Then we will cease reporting in % units. This is the procedure being used in all laboratories in New Zealand, and in the UK.

We do understand that the new units may require some changes to cumulative tracking databases, but the long term result of ensuring accuracy through traceability to the highest possible order of reference method is worth initial inconvenience. Such traceability is a legislative requirement in the European Union.