Using haemoglobin A1c (HbA1c) to diagnose diabetes may underestimate the prevalence, according to a study presented recently at the Endocrine Society’s meeting in New Orleans.

Researchers enrolled 9,000 adults aged 20 years or older without diagnosed diabetes. Using HbA1c of more than 6.4 per cent missed 73.1 per cent of cases of diabetes detected using an oral glucose tolerance test (fasting plasma glucose [FPG] at least 126mg/dl and two-hour post-glucose challenge [PGT] of at least 200mg/dl).

“The A1c test said these people had normal glucose levels when they didn’t,” said lead researcher Maria Mercedes Chang Villacreses, City of Hope’s Diabetes and Metabolism Research Institute in Duarte, California. In other words, HbA1c “significantly underestimated the true prevalence” of diabetes.

On the other hand, oral glucose tolerance tests confirmed normal glucose tolerance (FPG less than 100mg/dl and PGT less than 140mg/dl) in only 57.5 per cent of those defined using HbA1c (less than 5.7 per cent), so HbA1c “overestimated the true prevalence” of normal glucose tolerance.

“Our results indicated that the prevalence of diabetes and normal glucose tolerance defined solely by A1c is highly unreliable, with a significant tendency for under-estimation of the prevalence of diabetes and over-estimation of normal glucose tolerance,” Dr Chang Villacreses said.