Diabetes Home > Diagnosing Diabetes

The oral glucose tolerance test and the fasting plasma glucose test are common ways of making a diabetes diagnosis. The fasting plasma glucose test is the preferred test because of its convenience, although the oral glucose tolerance test is more accurate. Diagnosing this condition in pregnant women is based on plasma glucose values measured during the oral glucose tolerance test.

Diagnosing Diabetes: An Overview

In people with diabetes, the pancreas either produces little or no insulin, or the cells do not respond appropriately to the insulin that is produced. Glucose builds up in the blood, overflows into the urine, and passes out of the body. Thus, the body loses its main source of fuel even though the blood contains large amounts of glucose.
 
In pre-diabetes, blood glucose levels are higher than normal but not high enough to be characterized as diabetes.
 
The fasting plasma glucose test is the preferred test for making a diabetes diagnosis -- both type 1 or type 2 diabetes. It is most reliable when done in the morning; however, a diabetes diagnosis can be made after positive results with any one of four tests, with confirmation results from the same test on a different day:
 
  • A plasma glucose value of 126 mg/dL or more after a person has fasted for eight hours.
     
  • An oral glucose tolerance test (OGTT) plasma glucose value of 200 mg/dL or more in a blood sample taken two hours after a person has consumed a drink containing 75 grams of glucose dissolved in water. This test, taken in a laboratory or the doctor's office, measures plasma glucose at timed intervals over a three-hour period.
     
  • A random (taken any time of day) plasma glucose value of 200 mg/dL or more, along with the presence of diabetes symptoms.

 

  • A hemoglobin A1C level of 6.5 percent or higher.

 

Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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