Although your health care provider may suspect that you have diabetes because of your symptoms, the only sure way to tell is with blood tests.
Blood tests are used to diagnose both type 1 and type 2 diabetes, as well as gestational diabetes.

These blood tests may be repeated to confirm the diagnosis.

The blood tests are based on the fact that diabetes causes your blood glucose levels to be above normal some or all of the time. Your blood glucose levels may be high even though you haven’t eaten recently.

Random plasma glucose tests are the simplest way to detect diabetes. This test measures the amount of glucose in the blood at any given time and is done without fasting. If you have obvious symptoms of diabetes and the amount of glucose in your blood is 200 mg/dl or higher, you have diabetes. Symptoms of diabetes include frequent urination,  intense thirst,  blurred vision, unexplained weight loss, and extreme tiredness.

The preferred method for diagnosing diabetes is the fasting plasma glucose test. In diabetes, extra glucose remains in the blood, even after fasting. For this test, you will be asked not to eat or drink anything but water for at least 8 to 10 hours. Then, a sample of your blood is taken from a vein and the amount of glucose present in the blood is measured. Normally after fasting, the amount of glucose is less than 100 mg/dl.  But when the amount of blood glucose is greater than 126 mg/dl, diabetes is suspected. A firm diagnosis of diabetes is made when two fasting plasma glucose tests, done on different days, are over 126 mg/dl.

If your test results are greater than 100 mg/dl but less than 126 mg/dl, you may have impaired fasting glucose (IFG).

Some people also have impaired glucose tolerance (IGT), a condition in which blood glucose levels are higher than normal (140 mg/dl to 199 mg/dl) 2 hours after the start of an oral glucose tolerance test (GTT). If you have IFG and/or IGT, you may be diagnosed with pre-diabetes. 

This is not diabetes,  but sometimes occurs before diabetes develops. Some people with pre-diabetes never get diabetes. However, some of the same problems that result from having diabetes also occur in people with pre-diabetes.  If you have been diagnosed with prediabetes, you will want to have your blood glucose tested routinely and to watch for symptoms of diabetes. Also, you need to talk with your provider about reducing your risk of heart disease. Keeping your weight in the healthy range and exercising regularly will lower your chances of developing diabetes.

Certain pregnant women are at high risk for developing gestational diabetes. These are women who are 25 years of age or older, are overweight, have a parent or sibling with diabetes, or are Hispanic, American Indian, Asian, or African American.

Women who had gestational diabetes in a previous pregnancy have a greater risk of developing it again.

If you have any of these characteristics, you will be screened for gestational diabetes with a glucose challenge. This is done between the 24th and 28th weeks of pregnancy.

At this time, the hormones of pregnancy naturally begin to cause temporary insulin resistance that lasts until the baby is born. The glucose challenge helps you find out whether your body is able to overcome the insulin resistance on its own. You are given a glucose drink to finish at a certain time, without regard to eating. If the glucose in your blood 1 hour later is 140 mg/dl or above, you may have gestational diabetes. You will need a full glucose tolerance test for a firm diagnosis.

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Martha M. Funnell, MS, RN, CDE
Michigan Diabetes Research and Training Center
University of Michigan Medical School
Ann Arbor, Michigan

Robert M. Anderson, EdD
Michigan Diabetes Research and Training Center
University of Michigan Medical School
Ann Arbor, Michigan

Shereen Arent, JD
National Director of Legal Advocacy
American Diabetes Association

American Diabetes Association Complete Guide to Diabetes