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New Fasting Glucose Level Means More Prediabetes

New guidelines from the American Diabetes Association will place even more people into the “prediabetes” category. At least 20% more which, could mean 30 million people with “prediabetes.”

The ADA’s Follow-up Report on the Diagnosis of Diabetes Mellitus reduces the lower limit for the diagnosis of “impaired fasting glucose (IFG)” from 110 to 100 mg/dL, and redefines “normal” as a fasting plasma glucose (FPG) of less than 100 mg/dL (Diabetes Care 26[11]:3160-67, 2003).

Other diagnostic thresholds related to impaired glucose regulation that the ADA defined in 1997 remain the same: Diabetes is diagnosed at a FPG of 126 mg/dL or above, and the cutoffs for the 2-hour oral glucose tolerance test remain at 140-199 mg/dL for impaired glucose tolerance (IGT) and 200 mg/dL or greater for diabetes. As before, the diagnosis of diabetes requires a confirmatory test on a separate day (Diabetes Care 20[7]:1183-97, 1997).

Data acquired since 1997 suggest that the new cutoff of 100 mg/dL better correlates with a 2-hour plasma glucose value of 140 mg/dL and more accurately identifies the population at greatest risk for progression to type 2 diabetes in the absence of intervention. “This new FPG cutoff gave us the best combination of sensitivity and specificity,” ADA expert committee chair Dr. Saul Genuth told FAMILY PRACTICE NEWS.

In a recent review of data from around the world, only half or less of people with IFG by the old cutoff also had IGT and an even lower proportion (20%-30%) of those with IGT also had IFG (Diabet. Med. 19[9]:708-23, 2002).

The new cutoff is expected to increase the proportion of people with IFG who would also have IGT.

Some studies aimed at teasing out identifiable differences between the two groups have yielded conflicting data.

“At this point in time, there is no clear pathophysiologic basis for explaining IFG versus IGT. All we know is that both are at risk to go on to diabetes,” said Dr. Genuth, professor of medicine at Case Western Reserve University, Cleveland.

The ADA report notes that although the 2-hour oral glucose tolerance test probably will remain a more sensitive assay for detecting diabetes, the FPG is more reproducible, less costly, and likely to be more convenient in routine clinical practice.

Last year, the ADA and the U.S. Department of Health and Human Services issued a document on the prevention or delay of type 2 diabetes in individuals with IFG or IGT, urging physicians to screen individuals at high risk and to implement nutrition and physical activity interventions in those identified as having “prediabetes” (Diabetes Care 25[4]:742-49, 2002).

At that time, the ADA estimated that at least 16 million Americans aged 40-74 had prediabetes. While it is not clear exactly how many more individuals will now fall into the category with the new FPG cutoff, the total will likely exceed 20 million and perhaps even reach 30 million, according to ADA expert committee member Dr. John Buse, director of the Diabetes Care Center at the University of North Carolina at Chapel Hill.

Unlike the diabetes fasting blood glucose cutoff of 126 mg/dL, which was derived from its correlation with development of retinopathy, the 100-mg/dL FPG cutoff does not specifically identify a threshold for cardiovascular disease, which is the cause of death in more than 70% of people with type 2 diabetes.

“It will be very important to know whether the prevention of progression from IFG to diabetes will also prevent the development of cardiovascular disease,” Dr. Genuth noted.

The answer to that could come from a long-term follow-up of participants in the landmark Diabetes Prevention Program, the major U.S. trial that achieved a 58% reduction in progression to type 2 diabetes among individuals with IGT who followed a strict diet and exercise regimen, and a 31% reduction among those who took metformin for 3 years (N. Engl. J. Med. 346[6]:393-403, 2002).

In the meantime, the new IFG definition should help focus attention yet again on the importance of lifestyle intervention both for preventing diabetes and for improving overall health, both physicians said. “If lowering this number a little puts more people into serious lifestyle intervention mode, then it will have accomplished an enormous amount for the health of the country,” Dr. Genuth said.