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Using A1c to Define the Metabolic Syndrome

Jun 2, 2010

Using A1c instead of FPG to define MetS is feasible. It also identifies individuals with increased cardiovascular risk….

This study was done to compare the use of glycosylated hemoglobin (A1c) and fasting plasma glucose (FPG) to define the metabolic syndrome (MetS). 

The metabolic syndrome (MetS) describes the clustering of closely related cardiovascular risk factors. The definition of MetS, proposed in 2001 by the National Cholesterol Education Program (NCEP) Expert Panel, was later modified in accordance with the revised definition of impaired fasting glucose from the American Diabetes Association (ADA) in 2004. Recently, a unified definition of MetS was proposed jointly by several organizations in 2009 whilst ADA has proposed the use of A1c in the definition of diabetes and the category of increased diabetes risk (which also includes impaired fasting glucose and impaired glucose tolerance) in 2010. It was investigated whether A1c can be used instead of fasting plasma glucose (FPG) in identifying individuals with MetS. 

After excluding pregnant women and subjects with missing data in BMI, A1c, and the five components of MetS, there were 3,551 and 3,412 participants aged ≥20 years in NHANES 1999-2002 and 2003-2006 respectively who had fasted for 8-24 hours. In NHANES 2003-2006, the use of A1c alone resulted in a lower percentage of people meeting the glycemic criteria of MetS compared to the use of FPG alone, with 74.9% agreement between these two definitions. The use of A1c alone also resulted in a lower prevalence of MetSv compared to the use of FPG alone. 

The controversy regarding the definition of MetS has been addressed recently in a joint scientific statement. A1c reflects the average blood glucose level over several months and its measurement does not require a fasting blood sample. In this study, it was demonstrated that there was good agreement between A1c and FPG in identifying individuals with MetS, despite only a moderate agreement (~75%) between A1c and FPG in defining raised blood glucose.

The components of MetS are inter-correlated and so, a certain degree of inaccuracy or fluctuation in one component is tolerated and does not result in misclassification. The agreement between A1c and FPG in the definition of MetS is good in different subgroups. We can therefore confidently conclude that using A1c instead of FPG to define MetS is feasible. This is true at least for Americans, based on the most up-to-date data on a nationally representative sample of Americans, and confirmed using historical data. It remains to be seen if our conclusions are also applicable to Asians, among whom the prevalence of raised blood glucose is likely to be different.

The current cut point of A1c identifies a slightly smaller group of people as having MetS. However, it also identifies subjects at high risks for cardiovascular diseases, even in those without diabetes, when the consensus criteria in 2009 are used to define MetS. Whether A1c results in better risk stratification needs to be investigated in large prospective studies.

Diabetes Care, May 2010