A screening test for diabetes mellitus (DM), in which plasma glucose levels are measured after the patient consumes an oral glucose load. In screening patients for type 2 DM, measuring fasting plasma glucose levels generally is preferable to an OGTT because the fasting blood test is simpler, cheaper, and better tolerated by patients. An OGTT reveals type 2 DM when plasma glucose levels exceed 200 mg/dl 2 hr after drinking a 75-g glucose load. Plasma glucose levels between 140 mg/dl and 199 mg/dl suggest impaired glucose tolerance.