International Standardisation of Glycosylated Haemoglobin Level Reporting

There are multiple ways to monitor diabetes including the measurement of glucose, C-peptide, and glycosylated haemoglobin levels.

Haemoglobin (Hb) is found in the red blood cells and carries oxygen around the body. It exists as multiple types with the majority (90%) being haemoglobin A. Glucose binds to the haemoglobin A, forming glycosylated haemoglobin (HbA_1c). Since red blood cells have a life span of around 120 days HbA_1c can be used to measure long term blood sugar level (BSL) control for the preceding 1-3 months.

It is generally recommended that the HbA_1c level target for diabetic patients should be <7% of total haemoglobin. This target should be tailored to the individual patient as it may be higher in patients who have other co-morbidities, are elderly, have a limited life expectancy, or who are at higher risk of hypoglycaemia.

In Australia HbA_1c levels were reported prior to July 2011 as a percentage of total haemoglobin in National Glycohemoglobin Standardisation Program (NGSP) units with a usual reference range of <6.0% for non-diabetics and <7.0% for diabetics, but this is not the method of reporting in all countries. In order to standardise the way HbA_1c is reported it was decided to change the reporting of HbA_1c from NGSP units to the new Système International (SI) units which are reported as mmol/mol (i.e. millimoles of HbA_1c per mole of total haemoglobin) as per the International HbA_1c Consensus Committee recommendations.

Conversion tables are readily available and the National Prescribing Service provide a freely accessible, interactive, HbA_1c unit converter tool on their website. This converter tool may be accessed by patients as well as health care professionals.

Under the new system of reporting a HbA_1c level of 7% would equate to 53 mmol/mol. The table below demonstrates the HbA_1c level in the old units (NGSP) and in the new units (SI).

Table 1. Conversion between HbA_1c level as percentage and as mmol/mol.

This change has been made in order to ensure that HbA_1c is reported in the same way internationally. It is considered that reporting using SI units is more scientifically valid and removes the possible confusion that may arise between HbA_1c levels being reported as a percentage and blood glucose values reported in mmol/L.

In order to reduce confusion experienced by health care professionals and patients it was agreed that for the two years following the introduction of the international standardisation of HbA_1c in July 2011, the results would be displayed as both percentage and mmol/mol. After the two year dual reporting period has elapsed, the results will be reported as SI units only.

The change to HbA_1c reporting does not change the frequency of HbA_1c testing or the target HbA_1c, it only changes how the results are reported. For most people the new target HbA_1c level is now 53 mmol/mol or less. Healthcare professionals and patients alike should familiarise themselves with the new reporting system to avoid confusion once the two year dual reporting period ends in July this year.