Though symptoms can point to diabetes, the only way for a physician to diagnose diabetes is with blood tests. When physicians determine that a blood test result is outside of normal range, they may order repeat tests to verify results or additional tests to determine the underlying causes behind the abnormality. Common tests for diagnosing diabetes include:

• Random plasma glucose test. This test may be conducted as a part of a routine physical examination. Glucose levels above 200 milligrams per deciliter (mg/dL) accompanied by symptoms will lead to a diabetes diagnosis. A fasting glucose test may be obtained to confirm the results. 
  
  
• Fasting blood glucose test (FPG). Normal fasting blood glucose is below 100 mg/dL. Glucose levels of 126 mg/dL or above will lead to a repeat test. If the results of the second test are the same, the patient will most likely be diagnosed with diabetes.
  
  
• Oral glucose tolerance test (OGTT). Normal blood sugar is below 140 mg/dL two hours after consuming a glucose-rich drink. If the level rises to 200 mg/dL or above, the person will most likely be diagnosed with diabetes.

The American Diabetes Association (ADA) recommends that anyone age 45 or older have a FPG, especially if overweight or obese. If results are normal, the patient should be retested again every three years. Patients diagnosed with prediabetes should be checked for type 2 diabetes every one to two years.

For overweight adults younger than 45, a physician may recommend testing if any other risk factor for diabetes is present. 

Children and adolescents who are at risk should be screened for diabetes every two years after the age of 10, or at the onset of puberty if it occurs at an earlier age.