The GTT is usually administered after an abnormal urine test. Doctors use glucose tolerance testing to monitor the amount of glucose in the patient's blood at a given moment in time and to see if their body reacts properly in response to the glucose. If the glucose levels rise drastically and don't fall back down this indicates that there is a high chance that the patient has diabetes. The insulin test can determine the difference between Type 1 and Type 2 diabetes; if the levels of insulin in the blood are high, the patient has Type 2 diabetes, and if there is no insulin in the blood the patient has Type 1.