Blood sugar fasting and postprandial measure the amount of sugar (glucose) in a person's blood. The fasting plasma glucose test is done on an empty stomach. For the eight hours before the test, the person must fast (nothing to eat or drink, except water). The fasting plasma glucose test is done on an empty stomach. For the eight hours before the test, the person must fast (nothing to eat or drink, except water). The two-hour postprandial blood glucose test measures the amount of glucose in plasma after a person eats a specific meal containing a certain amount of sugar. Blood sugar tests are done primarily to diagnose and evaluate a person with diabetes mellitus.