Hello, it is not common for healthy individuals to have that high blood glucose levels. Actually a single measurement of blood glucose (by a laboratory) above 200mg/dL accompanied by symptoms of diabetes is sufficient for the diagnosis of diabetes mellitus according to guidelines. You hadn t mentioned symptoms. Were there any? The classical triad is body weight loss, thirst and increased urine production all of which imply dehydration due to glycosuria. If you had no symptoms then an oral glucose tolerance test with 75 grams of glucose would be appropriate. This test will establish or rule out the diagnosis of diabetes. Please note that if any of your father, mother or brothers/sisters had diabetes then your chances for diabetes are higher. Also keep in mind that self measurements are not as accurate as laboratory measurements. In any case blood sugars even slightly above 200 mg/dL are suspicious. If you need further clarifications, please ask me again! Kind Regards!