The Oral Glucose Tolerance Test: An Old but Irreplaceable Test to Evaluate Glucose Metabolism and Cardiovascular Risk

The International Diabetes Federation estimates that near 285 million people have known type 2 diabetes: their number will probably double within 20 years (Shaw et al., 2010). Furthermore, worldwide, the number of persons with prediabetes, defined as impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) (Genut et al., 2003), is estimated to be 314 million and is expected to be 418 million in 2025 (Garber et al., 2008). This scenario, amplified by the fact that several subjects do not know they have diabetes (Garber et al., 2008), has a predictable consequence: as the prevalence and progression to type 2 diabetes continues to increase and the afflicted population’s age rises, the associated complications of diabetes inevitably will emerge as a major public health care issue. In 2007, for example, the direct and indirect costs related to diabetes, diabetes complications and general medical care amounted to $ 174 billions in the United States (American Diabetes Association [ADA], 2008). Thus, the advantage not only to diagnose, but also to recognize as soon as possible subjects at high risk to develop type 2 diabetes, is evident. If, on one hand, the magnitude of morbidity and early mortality attributable to diabetes has been clearly shown (ADA, 2008), on the other hand a growing body of evidence indicates that earlier detection and consequent earlier treatment of hyperglycaemia and related metabolic abnormalities may be beneficial (DREAM Trial Investigators, 2006; Knowler et al., 2002). In fact, early detection and treatment of subjects with prediabetes has the potential of reducing or delaying the progression to diabetes (DREAM Trial Investigators, 2006; Gillies et al., 2007; Knowler et al., 2002) and related cardiovascular disease (Chiasson et al., 2003; Ratner et al., 2005). The risk associated with progression to diabetes and cardiovascular complications increases along a continuum, rather than being threshold-dependent, and occurs at much lower glucose levels than those required to diagnose diabetes. Consequently, relying exclusively on diabetic glucose level may delay treatment (Bergman, 2010), as we need to maximize our efforts in diabetes prevention and early disease management. How can we identify not only unknown diabetics but, above all, those subjects with glucose levels not yet in the diabetic range, who do instead mostly need preventive interventions?