Is haemoglobin A1c a step forward for diagnosing diabetes?
(Published 10 November 2009)
Cite this as: BMJ 2009;339:b4432
- Eric S Kilpatrick, consultant in chemical pathology1,
- Zachary T Bloomgarden, clinical professor of medicine2,
- Paul Z Zimmet, director emeritus3
- 1Department of Clinical Biochemistry, Hull Royal Infirmary and Hull York Medical School, Hull HU3 2JZ
- 2Department of Medicine, Mount Sinai School of Medicine, New York, USA
- 3Baker IDI Heart and Diabetes Institute, Caulfield South 3162, Australia
- Correspondence to: Eric S Kilpatrick
Currently, diabetes is diagnosed by measuring plasma glucose concentration fasting (threshold ≥7 mmol/l) or after a oral glucose tolerance test (≥11.1 mmol/l). However, an international expert committee comprising members appointed by the American Diabetes Association, the European Association for the Study of Diabetes, and the International Diabetes Federation recently recommended replacing these tests with one for glycated haemoglobin A1c (HbA1c).1 The committee stated that type 2 diabetes should be diagnosed in anyone with a confirmed HbA1c value ≥6.5% (48 mmol/mol) without glucose testing, although the glucose criteria will continue to be used in people in whom measurement of HbA1c may be inappropriate. Measurement of HbA1c has several advantages over glucose but its exclusive use could present problems, as we discuss here.
Advantages of haemoglobin A1c
The expert committee document gave no specific reasons for dispensing with glucose criteria in favour of HbA1c, but it did highlight many of the advantages of using HbA1c (table⇓). These include the undoubted benefits of requiring a single blood sample and being able to test in the non-fasting state. The day to day variability of HbA1c within an individual is also smaller than that of fasting glucose and considerably less than glucose concentration after a glucose tolerance test (coefficient of variation 3.6% v 5.7% v 16.6% in one study2), so repeated measurements should be more consistent with HbA1c. There is also the argument that, by giving an estimate of glycaemia over the preceding few weeks or months, HbA1c could provide a more complete view of glycaemia than a single fasting glucose measurement or the “artificial” conditions of a glucose tolerance test, which …
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