Change of HbA1c Units to mmol/mol
From October 2011 HbA1c results reporting has changed. This followed the recommendations of the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) call to adopt the same measurement to make it easier to compare HbA1c results between studies and laboratories worldwide. Hence this change is due to the development of a standardised reference method for measuring HbA1c and also a move to using the Standard International (SI) unit of millimoles of HbA1c per mole of Hb (mmol/mol) .
This was first adopted in June 2009. During the first two years until May 2011, results were being reported in both units as %age and mmol/mol, so as to enable health professionals and patients to get used to the new units. This was then further postponed for 6 months.
When HbA1c results are expressed as %age haemoglobin, the equation describing the relationship is:
IFCC/HbA1c (mmol/mol) = [DCCT/HbA1c %age – 2.15] × 10.929
|DCCT aligned HbA1c (%age)||New IFCC HbA1c (mmol/mol)|
In a busy diabetes clinic converting these units can be a challenge.
An easy way to get an idea of the HbA1c values is to follow the rule of 2s. For eg. 7% would be 7-2 =5 and 5-2=3: 53 mmol/mol or 9% would be 9-2=7 and 7-2=5: 75 mmol/mol.
Another useful way to remember these conversions is that 7.0% is equivalent to 53 mmol/mol. Every %age increase or decrease thereafter is equivalent to 11 mmol/mol. For eg. 8% is 64 mmol/mol and 9% is 75 mmol/mol. These methods work well for HbA1c from 4% up to 13% which is the case for the majority of patients with diabetes presenting to our clinics.
A number of international societies are moving to use HbA1c to diagnose diabetes. An HbA1c of 48 mmol/mol (6.5%) is recommended as the cut point for diagnosing diabetes. However, a value of less than 48 mmol/mol (6.5%) does not exclude diabetes diagnosed using glucose tests.
- Barth JH et al. Consensus meeting on reporting glycated haemoglobin and estimated average glucose in the UK: report to the National Director for Diabetes, Department of Health. Ann Clin Biochem 2008; 45: 343-4