Impaired glucose tolerance (IGT) was considered a class in the previous WHO classification but is now categorized as a stage in the natural history of disordered carbohydrate metabolism. A stage called 'impaired fasting hyperglycemia' or impaired fasting glycemia (IFG) or 'non-diabetic fasting hyperglycemia' is now recognized as these people also appear to be at greater risk for progression to diabetes and macrovascular disease, although prospective data are sparse and early data suggest a lower risk of progression than IGT (21). IFG refers to fasting glucose concentrations which are lower than those required to diagnose diabetes mellitus but higher than the 'normal' reference range.
IGT and IFG are not clinical entities in their own right (mostly in the absence of pregnancy for
IGT), but rather risk categories for future diabetes and/or cardiovascular disease (22,23). IGT and IFG represent impaired glucose regulation which refers to a metabolic intermediate between normal glucose homeostasis and diabetes. Individuals who meet criteria for IGT or IFG may be euglycemic in their daily lives as shown by normal or near-normal glycated hemoglobin levels (2). IGT is often associated with the metabolic syndrome (insulin resistance syndrome) (24). An individual with a fasting plasma glucose concentration of 6.1 mmol/l (110 mg/dl) or greater (whole blood 5.6 mmol/l; 100 mg/dl) but less than 7.0 mmol/l (126 mg/dl) (whole blood 6.1 mmol/l; 110 mg/dl) is considered to have impaired fasting glycemia (IFG). If an OGTT is performed, some individuals with IFG will have IGT. Some may have diabetes but this cannot be determined without an OGTT. If resources allow, it is recommended that those with IFG have an OGTT to exclude diabetes (2).
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