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► Health Information LibraryCare GuidesStep 4: Diagnosing diabetes

Step 4: Diagnosing diabetes


How does a doctor identify diabetes? Once symptoms appear, type 1 diabetes rarely goes undiagnosed for more than a few weeks. The most important clue is frequent urination, sometimes accompanied by feeling thirsty all the time, poor weight gain, or flu-like symptoms. If you and your doctor suspect diabetes, testing is necessary to confirm the diagnosis. There are two tests done to check for blood sugar levels.

Random blood glucose test

A random blood, or plasma, glucose test can be done at any time, regardless if you have eaten or not. A blood sample is drawn from the arm or the back of the hand and analyzed for glucose level. If the glucose level is greater than or equal to 200 mg/dL, it generally means the person has diabetes. Glucose in the urine is strong additional evidence.

However, a random blood glucose test below this level does not eliminate the possibility of diabetes.

Fasting plasma glucose (FPG) test

This is the recommended diagnostic test for diabetes. "Fasting" means that no food can be eaten for 8 - 12 hours prior to the test -- only water may be consumed. A blood sample is drawn in the same manner as the random test.

If there is any doubt that a patient truly has diabetes, two or more FPG tests may be done on different days to verify the results.

What does the FPG test reveal?

Normal fasting glucose levels are lower than 100 mg/dL. Patients whose glucose is between 100 mg/dL and 125 mg/dL have "impaired fasting glucose" and are at risk for developing type 2 diabetes, although they do not yet have diabetes.

When fasting glucose levels are 126 mg/dL or higher, the patient has diabetes. Sometimes, at the moment of diagnosis, people with type 1 diabetes have glucose levels that are "off the charts," often higher than 400 mg/dL. It is very important to see a doctor right away if you suspect diabetes.


American Diabetes Association. Standards of medical care in diabetes--2009. Diabetes Care. 2009 Jan;32 Suppl 1:S13-61.