Diabetes Mellitus - Diagnosis 

The diagnosis of diabetes is made when people have abnormally high levels of sugar in the blood. Blood sugar levels are often checked during a routine physical examination. Checking the levels of sugar in the blood annually is particularly important in older people, because diabetes is so common in later life. People may have diabetes, particularly type 2 diabetes, and not know it. Doctors may also check blood sugar levels in people who have symptoms of diabetes such as increased thirst, urination, or hunger. Doctors may also check blood sugar levels in people who have disorders that can be complications of diabetes, such as frequent infections, foot ulcers, and yeast infections.

To measure the blood sugar levels, a blood sample is usually taken after people have fasted overnight. However, it is possible to take blood samples after people have eaten. Some elevation of blood sugar levels after eating is normal, but even after a meal the levels should not be very high. Fasting blood sugar levels should never be higher than 126 mg/dL. Even after eating, blood sugar levels should not be higher than 200 mg/dL.

Doctors can also measure the level of a protein in the blood, hemoglobin A1C (also called glycosylated or glycolated or hemoglobin). A1C levels below 5.7 are within normal range, 5.8 - 7.0 is suggestive of glucose intolerance or diabetes, and greater then 7.0 is consistent with true diabetes.  Glycosylated hemoglobin forms when the blood has been exposed to high blood sugar levels over a period of time. Doctors do not usually use this test to diagnose diabetes, but the test can help confirm the diagnosis when blood sugar levels are not extremely high. The test demonstrates long-term trends in blood sugar levels.

Another kind of blood test, an oral glucose tolerance test, may be done in certain situations, such as in routine screening of pregnant women for gestational diabetes (see Pregnancy Complicated by Disease: Gestational Diabetes) or in older people who have symptoms of diabetes but normal glucose levels when fasting. However, it is not routinely used for testing for diabetes, including in pregnant women at very low risk. In this test, people fast, have a blood sample taken to determine the fasting blood sugar level, and then drink a special solution containing a large, standard amount of glucose. More blood samples are then taken over the next 2 to 3 hours and are tested to determine whether the level of sugar in the blood rises abnormally high.