Experts caution against using HbA1c alone to diagnose diabetes

Oral glucose tolerance test is much better at case-finding, study shows

Diagnosing type 2 diabetes on the basis of a single HbA1c test will miss nearly three-quarters of people with the disease, according to results announced at a major conference.

blood test

Presenting to delegates at the US Endocrine Society’s annual meeting in New Orleans, Dr Maria Chang Villacreses said that compared to an oral glucose tolerance test (OGTT), 73% of people with diabetes would be missed if diagnoses were made on the basis of an HbA1c test alone.

In the study of 9000 participants in the 2005-14 National Health and Nutrition Examination Survey aged over 20 years without a prior diagnosis of diabetes, Dr Chang Villacreses compared their results from an OGTT with those of an HbA1c test.

Using those classified as having diabetes by OGTT (fasting blood glucose >7mmol/L, two-hour post-challenge plasma glucose >11.1mmol/L) as a reference, the sensitivity of HbA1c for detecting diabetes was just 27%.

The criterion used for diagnosing diabetes with an HbA1c test was >48mmol/mol (6.5%), said Dr Chang Villacreses of City of Hope's Diabetes and Metabolism Research Institute in California, US.  

"Based on our findings, [Hb]A1c should not be solely used to determine the prevalence of diabetes," she said.

In Australia, diagnosis can be made either by two HbA1c measurements >48mmol/mol or two fasting blood glucose levels >7mmol/L made on different days, or by a single OGTT.

"Our results indicated that the prevalence of diabetes and normal glucose tolerance defined solely by [Hb] A1c is highly unreliable, with a significant tendency for underestimation of the prevalence of diabetes and overestimation of normal glucose tolerance," Dr Chang Villacreses concluded.

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