A series of urine and blood tests are used to diagnose all types of diabetes.
Fasting Blood Glucose (FPG) Test
This is one of the most common and preferred tests; it measures blood glucose in a person who has not eaten anything for at least eight hours. Both diabetes and prediabetes can be diagnosed this way. A blood glucose level of 126 milligrams per deciliter (mg/dL) or higher indicates diabetes.
Random (Non-fasting) Plasma Glucose Test
This measures blood glucose without fasting. This test, along with an assessment of symptoms, is used to diagnose diabetes but not prediabetes. A blood glucose level of 200 mg/dL or higher indicates diabetes.
Oral Glucose Tolerance Test (OGTT)
This blood glucose test, administered after fasting for at least eight hours, is performed two hours after drinking a glucose-containing beverage. It is also common to test blood prior to the glucose drink and then every 30 to 60 minutes afterwards for up to three hours. Both diabetes and prediabetes can be diagnosed this way, and it is routinely used to screen for gestational diabetes. A blood glucose level of 200 mg/dL or higher (two hours after drinking a beverage containing 75 grams of glucose) indicates diabetes. Because glucose levels are normally lower during pregnancy, gestational diabetes is based on slightly different numbers: 155 mg/dL two hours after drinking the glucose beverage.
Glycated Hemoglobin (A1C) Test
The higher your blood sugar levels, the more hemoglobin you’ll have with sugar attached. The A1C test measures the percentage of blood sugar attached to hemoglobin, the oxygen-carrying protein in red blood cells. The goal of the A1C test is to measure your average glucose levels for two to three months. A long-term average can be more accurate than a one-time test. An A1C level of 6.5 percent or higher on two separate tests indicates you have diabetes.
Although this test alone cannot diagnose diabetes, a urine analysis for abnormal levels of glucose and ketones from the breakdown of fat is often used as part of an overall diagnosis.