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HbA1c assay misclassifies adults with type 2 diabetes in Mexico City study
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An analysis of the Mexico City Diabetes Study revealed that a large proportion of adults identified as having prediabetes or type 2 diabetes by the HbA1c assay had normal glucose tolerance when undergoing an oral glucose tolerance test, according to findings published in the Journal of the Endocrine Society.
“Among the recognized limitations, the ADA states that HbA1c has low sensitivity,” Clicerio González Villalpando, MD, MACP, of the diabetes and cardiovascular research unit of the National Institute of Public Health in Mexico City, and colleagues wrote. “Our research findings confirm this limitation. The poor specificity observed in our data is another serious obstacle. We identified up to 45.5% of individuals who have an elevated HbA1c coexisting with normal OGTT.”
González Villalpando and colleagues analyzed results of 854 adults without known diabetes at baseline participating in the Mexico City Diabetes Study who underwent both an OGTT and an HbA1c assay on the same day during the 2008 phase to determine diabetes status. Participants also completed interviews and exams, in which height, weight, and waist and hip circumference were measured, as well as blood pressure, lipid profile and other biochemical measurements. Type 2 diabetes and prediabetes were diagnosed according to American Diabetes Association criteria.
Based on OGTT criteria, 81 participants (9.4%) had new-onset type 2 diabetes, 261 participants (30.5%) had prediabetes and 512 (59.9%) had normal glucose tolerance. Weight, BMI, waist circumference, systolic and diastolic BP and triglycerides were all higher in the type 2 diabetes group.
However, when defining diabetes by HbA1c, numbers were markedly higher, according to researchers: 451 participants (52.8%) had type 2 diabetes, 281 participants (32.9%) had prediabetes and 122 (14.2%) had normal glucose tolerance. Additionally, when diagnosing by HbA1c assay, there were no between-group differences for weight, BMI, waist circumference, BP or triglycerides.
When using only fasting glucose as the diagnostic criterion, numbers again differed — from 81 diagnosed with type 2 diabetes by OGTT to 35 (4%). The variables found to be different between OGTT diagnostic groups were the same among those identified by fasting glucose alone, according to researchers.
Researchers also compared participants with a normal OGTT coexisting with an elevated HbA1c (n = 184) to those with a normal OGTT and a normal HbA1c value (n = 220) and found no between-group differences for height, weight, BMI, BP or lipid measurements.
“It is known that certain individuals are prone to higher glycosylation rates, through somewhat incompletely understood mechanisms,” the researchers wrote. “This phenomenon has been identified in the African-American group. Our finding suggests that this characteristic occurs in the Mexican population as well and possibly, even at higher frequency than what can be estimated, based on the findings obtained in the African-American population.”
The researchers noted that the findings might not be representative of the entire Mexican population and that there could be cases of true type 2 diabetes coexisting with a high tendency for excessive glycosylation.
“It is unfortunate that HbA1c is not the optimal tool to diagnose [type 2 diabetes] or [prediabetes],” the researchers wrote. “In our population, the proportion of subjects with an elevated glycosylation pattern is high enough that we should warn clinicians and the health care system about the risks of a costly misclassification.” – by Regina Schaffer
Disclosures: The authors report no relevant financial disclosures.
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