Glycated haemoglobin A 1c and diagnosis of diabetes. The test has finally come of age

Andrew Day ¹ highlighted that the American Diabetes Association has formally endorsed glycated haemoglobin (HbA_1c) ≥6.5% (48 mmol/mol) as an alternative to glucose-based diagnosis of diabetes, ² a position supported by the World Health Organization, ³ and more recently endorsed in the UK (see http://www.acb.org.uk/docs/Article%20Summary%20Use%20of%20HbA1c%20WHO%20guidance%20251111%5B1%5D.pdf, accessed 19 January 2012).

In New Zealand, this position has been formally endorsed by the New Zealand Society for the Study of Diabetes (NZSSD) from 3 October 2011. New Zealand has taken the opportunity to coordinate this recommendation with the adoption of using exclusively molar units for the reporting of HbA_1c (following a two-year period of dual reporting, like the UK), but also with the diagnostic cut-off rounded up to ≥50 mmol/mol (6.7%), and repeated on a second occasion in asymptomatic individuals (http://www.nzssd.org.nz/hba1c.html, accessed 19 January 2012). Individuals with HbA_1c in the range 41–49 mmol/mol are labelled as having ‘dysglycaemia’ or abnormal glucose tolerance and with the recommendation for retesting in 6–12 months and also implementation of cardiovascular risk management.

Part of the NZSSD rationale for rounding of HbA_1c was to make the molar units more memorable, although in addition, to maximize the specificity for the diagnosis of diabetes. It may be argued, as Andrew Day¹ indicates, that sensitivity is being compromised by adoption of HbA_1c as a diagnostic test, and especially at a higher concentration still, and that cases of diabetes will be missed. We would contend, however, that individuals with HbA_1c close to the cut-off (41–49 mmol/mol) will be retested in 6–12 months and will enter a lifestyle programme where cardiovascular risk factors will be appropriately addressed. Although they will not acquire the diagnostic label of diabetes, nor enter an annual programme of microvascular complications screening at that time, they are not really being ‘missed’. In addition, although glucose-based criteria remain valid, the NZSSD recommendations strongly favour HbA_1c as the diagnostic test in preference to oral glucose tolerance testing (OGTT) and as a consequence, New Zealand has witnessed a significant increase in HbA_1c testing and corresponding decline in OGTT requests. Furthermore, as Andrew Day ¹ also highlights, glucose-based criteria remain preferable wherever there may be abnormal red cell turnover or haemoglobinopathy.

We endorse the adoption of HbA_1c as a diagnostic test for diabetes, although it needs to be supported by explicit and unambiguous recommendations, such as those promulgated by NZSSD, together with supporting educational resources.

DECLARATIONS

Competing interests: None.

Funding: Not applicable.

Ethical approval: Not required.

Guarantor: CMF.

Contributorship: CMF and MJC prepared this manuscript.