From the Joslyn Diabetes Center (affiliated with Harvard Medical School)
Fasting Plasma Glucose –This blood test is taken in the morning, on an empty stomach. A level of 126 mg/dl or above, on more than one occasion, indicates diabetes.
Casual or Random Glucose - This blood test can be taken anytime during the day, without fasting. A glucose level of 200 mg/dl and above may suggest diabetes.
If any of these test results occurs, testing should be repeated on a different day to confirm the diagnosis. If a casual plasma glucose equal to 200 mg/dl or above is detected, the confirming test used should be a fasting plasma glucose or an oral glucose tolerance test.