Impaired fasting glucose is a type of prediabetes, in which a person's blood sugar levels during fasting are consistently above the normal range, but below the diagnostic cut-off for a formal diagnosis of diabetes mellitus. Together with impaired glucose tolerance, it is a sign of insulin resistance. In this manner, it is also one of the conditions associated with metabolic syndrome.

Those with impaired fasting glucose are at an increased risk of vascular complications of diabetes, though to a lesser extent. The risks are cumulative, with both higher blood glucose levels, and the total amount of time it spends elevated, increasing the overall complication rate.

IFG can eventually progress to type 2 diabetes mellitus without intervention, which typically involves lifestyle modification. Those with impaired fasting glucose have a 1.5 fold increased risk of developing clinical diabetes within 10 years, when compared to the general population. Some studies suggest that without lifestyle changes, IFG will progress to clinically diagnosable diabetes in just under 3 years, on average.

Impaired fasting glucose is often, though not always, associated with impaired glucose tolerance, though it may occur in isolation, with such persons having a normal response to a glucose tolerance test.