The ideal or normal blood sugar range is between 70 and 100 milligrams per deciliter for a fasting blood glucose test. If it is a random blood sugar test, then the normal blood sugar reading should be lower than 125 milligrams per deciliter, notes the National Institutes of Health's MedlinePlus. A fasting test requires that the patient not eat for at least 8 hours prior to the blood test, while a random test has no requirements in regards to time of day.
The fasting test is especially useful for screening or diagnosing diabetes. Doctors also can monitor blood sugar levels in diabetic patient with these tests. When a patient has a fasting blood glucose reading result that is over 126 milligrams per deciliter, it is an indication of diabetes. A reading over 200 on a random blood glucose test means that a patient has diabetes, states the National Institutes of Health's MedlinePlus.
Both the random and fasting blood sugar tests can also be useful to screen for patients who are in the prediabetes stage, according to WebMD. For a fasting test, this range is from 100 to 125 milligrams per deciliter.
Although diabetes is a main cause of an abnormally high blood glucose reading, other medical conditions can also cause this, including pancreatic cancer and Cushing syndrome. Blood sugar levels can also be very low in some patients. Some conditions that can cause this include kidney disease and hypothyroidism, notes MedlinePlus.