The hallmark of diabetes is a high level of glucose, as well as other nutrients, in the blood stream. The challenge in diagnosing this disease is in determining what levels are normal, and what levels are abnormally elevated. The problem is not an easy one, since blood glucose levels normally rise temporarily after a meal. However, after studying blood glucose levels in many individuals and checking which ones go on to develop the diabetes complications, doctors have set specific criteria for the diagnosis of diabetes. Anyone who meets these criteria is at increased risk for long-term diabetic complications, such as damage to the nerves, kidneys and blood vessels, and should be treated.

The simplest and most common test for diabetes is to measure the blood glucose level before breakfast, or at least 8 hours after eating. This is called a fasting blood glucose level. If this value is higher than 126 mg/dl on two separate occasions, then that individual has diabetes. This test should not be done at home with a blood glucose meter. Rather, an individual should have blood drawn at a clinic or physician's office and the sample should be sent to a reliable laboratory.

In normal individuals, the blood glucose level usually stays under 140 mg/dl, even after a meal, and never goes over 200 mg/dl. In a person who has symptoms of diabetes, a blood glucose level of over 200 mg/dl at any time of the day indicates that her or she has diabetes. Taking a blood glucose reading at any time of the day other than before breakfast is called a random blood glucose test.

If the fasting and random glucose measurements do not provide clear-cut evidence of diabetes, then a more formal test may be needed. This test is called the glucose tolerance test. For this test, an individual quickly drinks about 8 ounces of liquid containing 75 grams of glucose. The blood glucose level is measured immediately before the test and again 30 minutes, 1 hour and 2 hours after drinking the liquid. Individuals with diabetes cannot store this large an amount of sugar and their blood glucose will rise to over 200 mg/dl during the test. Although this test is not usually needed, it is a very reliable way to test for diabetes. Those whose blood glucose values during the test are between 140 and 200 mg/dl do not have diabetes, but are said to have impaired glucose tolerance. Although they are not yet at risk for diabetic complications, about 25 percent will experience deterioration in their blood glucose control and will develop diabetes.

Another way to test for the level of blood glucose is by measuring the hemoglobin A1c (HA1c) level. When blood glucose levels are high, glucose attaches to hemoglobin molecules. The higher the blood glucose, the more hemoglobin molecules get "tagged" with glucose. This is an excellent test for estimating the average blood glucose level over the previous 6 weeks, and it is very helpful for monitoring a person's diabetes. However, this test is not accurate enough for diagnosing diabetes and it should not be used for this purpose.

Once diabetes is diagnosed, how do you know what type of diabetes a person has? Additional tests are usually not needed. If the person is over the age of 30 when diabetes is diagnosed, and is overweight, has a family history of diabetes, and the diabetes has come on slowly, then he or she almost certainly has type 2 diabetes. On the other hand, the child or young adult who develops diabetes, usually with a sudden onset and accompanied by weight loss, likely has type 1 diabetes. If there is doubt as to whether a person has type 1 or type 2 diabetes, specialized testing can be done to determine whether insulin secretion is present. Gestational diabetes, of course, only occurs during pregnancy to women who were not known to have the disease previously.

Finally, who should be tested for diabetes? Certainly, anyone with symptoms of diabetes should have their blood glucose levels checked. Individuals who are at increased risk for developing diabetes should also be tested, even if they do not have symptoms. This includes those who are over 40 and overweight, those who have a family history of diabetes, women who have had gestational diabetes, and those from ethnic groups with high rates of diabetes, such as Native Americans and those of Hispanic or African descent.