Glycosylated haemoglobin (HbA1c) is the most widely used measure of long-term glycaemic control in diabetes mellitus. HbA1c is produced by the glycosylation of haemoglobin at a rate proportional to the glucose concentration. The level of HbA1c therefore is dependent on:
A number of conditions can interfere with accurate HbA1c interpretation:
| Lower-than-expected levels of HbA1c (due to reduced red blood cell lifespan) | Higher-than-expected levels of HbA1c (due to increased red blood cell lifespan) |
|---|---|
| Sickle-cell anaemiaGP6D deficiencyHereditary spherocytosisHaemodialysis | Vitamin B12/folic acid deficiencyIron-deficiency anaemiaSplenectomy |
HbA1c is generally thought to reflect the blood glucose over the previous '3 months' although there is some evidence it is weighed more strongly to glucose levels of the past 2-4 weeks. NICE recommend
'HbA1c should be checked every 3-6 months until stable, then 6 monthly'.
The relationship between HbA1c and average blood glucose is complex but has been studied by the Diabetes Control and Complications Trial (DCCT). A new internationally standardised method for reporting HbA1c has been developed by the International Federation of Clinical Chemistry (IFCC). This will report HbA1c in mmol per mol of haemoglobin without glucose attached.
| HBA1c(%) | Average plasma glucose(mmol/l) | IFCC-HbA1c (mmol/mol) |
|---|---|---|
| 5 | 5.5 | |
| 6 | 7.5 | 42 |
| 7 | 9.5 | 53 |
| 8 | 11.5 | 64 |
| 9 | 13.5 | 75 |
| 10 | 15.5 | |
| 11 | 17.5 | |
| 12 | 19.5 |
From the above we can see that average plasma glucose = (2 * HbA1c) - 4.5