Impaired glucose tolerance: what are the clinical implications?

Impaired glucose tolerance (IGT) was standardized in 1979 by the National Diabetes Data Group and the World Health Organization as a risk factor for type 2 diabetes, replacing groups such as 'borderline' and 'chemical' diabetes. IGT was defined by a blood/plasma glucose value 2 h after a 75 g glucose load that was clearly abnormal but did not convey a risk of microangiopathy in those with non-diabetic fasting blood/plasma glucose levels. IGT is not uncommon, having a prevalence of 2-25% in adults. Determinants include age, obesity (total and central), family history of type 2 diabetes, physical inactivity and triglyceride levels. The main clinical significance of IGT is: (1) as a risk factor for type 2 diabetes, with 20-50% of individuals developing type 2 diabetes over 10 years; (2) as a risk factor for cardiovascular disease (CVD); and (3) as a component of the metabolic syndrome. IGT can be treated and this may prevent or delay progression to type 2 diabetes, though the effect of treatment on the risk of CVD is unknown.