1 Hour Oral Glucose Tolerance Test Predicts Risk for Type 2 Diabetes

The plasma glucose concentration at 1 hour during the oral glucose tolerance test (OGTT)with a plasma blood glucose greater then 155mg/dL. is a strong predictor of the risk for type 2 diabetes, according to the results of a new study.

The American Diabetes Association Consensus Statement has recommended metformin in addition to diet and exercise in individuals with impaired glucose tolerance or impaired fasting glucose to reduce their risk for diabetes. Currently, the OGTT is the standard method to identify individuals at risk for type 2 diabetes, but only 50% of participants with impaired glucose tolerance convert to diabetes within 10 years, and 40% of those who eventually develop diabetes have NGT at baseline. The metabolic syndrome as defined by the Adult Treatment Panel III criteria is also a predictor of later diabetes, and a combination of the OGTT and metabolic syndrome diagnosis may be a useful predictor of diabetes.

This is a longitudinal cohort study conducted as part of the San Antonio Heart Study to examine the predictive value of a model incorporating OGTT and metabolic syndrome for type 2 diabetes within 7 to 8 years.

"In longitudinal epidemiological studies, ~40% of subjects who develop type 2 diabetes have normal glucose tolerance (NGT) at baseline, indicating that there is a population of NGT subjects who are at risk for future type 2 diabetes," write Muhammad A. Abdul-Ghani, MD, PhD, from the University of Texas Health Science Center at San Antonio, and colleagues. "Recently, we demonstrated that subjects with NGT, despite having relatively low risk for type 2 diabetes, can be stratified into low- and high-risk categories based upon the relationship between their postload and fasting plasma glucose (FPG) concentrations."

The goal of this study was to evaluate the efficacy of 1-hour plasma glucose concentration and the metabolic syndrome to predict the risk for type 2 diabetes, using data from a study cohort of 1611 participants who were enrolled in the San Antonio Heart Study and who were free of type 2 diabetes at baseline. Plasma glucose and insulin concentrations were measured at 0, 30, 60, and 120 minutes during the OGTT. The investigators tested 2 models using glucose tolerance status, 1-hour plasma glucose concentration, and the presence of the metabolic syndrome for their ability to predict the risk for type 2 diabetes at 7 to 8 years of follow-up, based on results of an OGTT.

Using a cutoff point of 155 mg/dL for the 1-hour plasma glucose concentration during the OGTT, participants in each glucose tolerance group were stratified into low, intermediate, and high risk for type 2 diabetes. A model using 1-hour plasma glucose concentration, Adult Treatment Panel III criteria for the metabolic syndrome, and fasting plasma glucose level independent of 2-hour plasma glucose level was equally effective in stratifying participants without diabetes into low, intermediate, and high risk for type 2 diabetes. This model also identified a group of normal glucose-tolerant participants who were at very high risk for type 2 diabetes.

"The plasma glucose concentration at 1 h during the OGTT is a strong predictor of future risk for type 2 diabetes," the study authors write. "A plasma glucose cutoff point of 155 mg/dL and the ATP [Adult Treatment Panel] III criteria for the metabolic syndrome can be used to stratify nondiabetic subjects into three risk groups: low, intermediate, and high risk."

Practice Pearls:

• A 1-hour plasma glucose cutoff level of 155 mg/dL with the OGTT has maximal sensitivity and specificity to predict type 2 diabetes.
• A combination of the 1-hour plasma glucose level during the OGTT and the presence of metabolic syndrome is a useful predictor of diabetes.

Diabetes Care, Aug 2008

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