Making the Diagnosis
Diabetes is diagnosed with simple blood tests. Your doctor may diagnose diabetes if the level of glucose in your blood after 8 hours of fasting is 126 mg/dL (7.0 mmol/L) or higher. If your fasting blood glucose is between 110 mg/dL and 125 mg/dL (6.1 mmol/L and 6.9 mmol/L), then you may have a condition known as impaired fasting glucose or prediabetes, which may later develop into diabetes.
Diabetes is also diagnosed if a blood glucose level taken anytime of the day without regards to meals is 200 mg/dL (11.1 mmol/L) or higher, plus you have symptoms characteristic of diabetes (e.g., increase thirst, increase urination, unexplained weight loss). It is important to note that one high blood glucose reading does not mean you have diabetes. In most cases, at least 2 high blood glucose readings are required before your doctor will make a diagnosis, unless type 1 diabetes is suspected or the person is very sick. Your doctor may also examine your eyes for signs of damage to the blood vessels of the retina (back of the eye).
Doctors may also diagnose diabetes based on the results of an oral glucose tolerance test (OGTT). With this test, a person fasts and then is given some glucose, after which their blood glucose is tested at various times to determine how quickly the sugar travels out of the bloodstream and into cells. In an OGTT, if the blood glucose level taken 2 hours after 75 g of glucose is given measures 200 mg/dL (11.1 mmol/L) or higher, the doctor may make a diagnosis of diabetes.