Although symptoms can point to diabetes, the only way for a physician to diagnose it is with blood tests. A physician who determines that a test result is outside of the normal range may order repeat glucose tests to verify results or additional tests to determine the causes of the abnormality. Common tests for diagnosing diabetes include:

• Fasting plasma glucose test (FPG). This screening test for diabetes is often conducted when people have a routine physical examination. A blood sample is taken after the patient has not eaten for at least 10 hours. Normal fasting glucose is below 100 milligrams per deciliter of blood (mg/dL).
  
  
• Glucose challenge test. Many pregnant women receive this test to screen for gestational diabetes. The patient is given 50 grams of a sugary solution to drink. A blood sample is drawn an hour later. If results are abnormal, an oral glucose tolerance test is used to confirm diagnosis.
  
  
• Oral glucose tolerance test (OGTT). This may be used in diagnosing gestational or other forms of diabetes. Preparation includes eating a high-carbohydrate diet for a few days and then fasting for at least 10 hours. Normal glucose is below 140 mg/dL two hours after consuming a sugary drink.

The American Diabetes Association (ADA) recommends that anyone (female or male) age 45 or older have a fasting blood glucose test, especially if overweight or obese. If results are normal, the patient should be retested again every three years. A patient diagnosed with prediabetes should be checked for type 2 diabetes every one to two years. For overweight patients younger than 45, a physician may recommend testing if any other risk factor for diabetes is present.