Patients are required to fast for 8 to 16 hours before a glucose tolerance test in order to eliminate the effects of recent meals on blood sugar levels, according to WebMD. This provides an accurate reading of the patient's baseline blood sugar.
The oral glucose tolerance test requires multiple blood sugar readings over a period of several hours, explains WebMD. For an accurate result, the first reading must be free of any interference from recent food intake. After the first reading is taken, the patient consumes 75 or 100 milligrams of sugar, usually in the form of a sweetened drink. Subsequent readings are taken at intervals to track blood sugar levels over time.
An abnormally high initial reading, if no food has been consumed immediately prior to the test, can be a sign of diabetes or insulin resistance, according to WebMD. Readings taken later in the test may be compared with the initial reading to establish a pattern that indicates either a normal sugar tolerance, an abnormal resistance to insulin, which is distinct from diabetes, or the presence of diabetes. Without an accurate picture of the patient's normal blood sugar levels given in the first reading, later readings cannot be interpreted properly.