What tests are used to diagnose diabetes?

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The two most common tests to diagnose diabetes are the fasting plasma glucose (FPG), in which blood sugar levels are measured after you’ve fasted for a specified time; and the oral glucose tolerance test (OGTT), in which you drink a sugary liquid and then blood sugar levels are tested several times to see how well glucose moves into cells. The American Diabetes Association (ADA) recommends you be tested with either option twice, at different times, to confirm a diagnosis.

For the FPG test, you will be tested after not eating (fasting) for a specific amount of time. Fasting blood sugar levels:
  • normal: 70-99 milligrams per deciliter (mg/dL)
  • prediabetes: 100-125 mg/dL
  • diabetes: 126 mg/dL or above
If your OGTT score is 140-199 mg/dL you're prediabetic; above that is diabetes.

If your initial test for diabetes is normal, you should be retested every three years, more often depending on your risk factors and your healthcare professional's recommendations.
One of the ways to screen for diabetes in symptomless individuals is to measure blood sugar levels after fasting; more than 126 milligrams of blood sugar per deciliter of blood (126 mg/dL) is considered an abnormal value. Another way to screen for or further diagnose diabetes is to give an oral glucose tolerance test. A series of blood samples is taken before and after swallowing a liquid that contains glucose. A blood-sugar reading that remains at or above 200 mg/dL two hours after drinking the glucose solution indicates inadequate production of insulin and, therefore, diabetes. Values between 127 and 199 mg/dL indicate a "glucose intolerance" state.

Glucose levels have to be abnormal on more than one occasion before a diagnosis of glucose intolerance or diabetes is made. Most persons detected with abnormal glucose tolerance do not proceed to frank diabetes, especially if they pay attention to maintaining recommended weight, diet and exercise.
There are several tests that can be used to diagnose diabetes. First is a random blood glucose level of 200 mg/dl or greater in combination with classic symptoms such as thirst, excessive urination, weight loss or blurry vision. This test must then be confirmed on a different occasion. Other tests include a fasting glucose level at or above 126 mg/dL, a hemoglobin A1C level of 6.5 percent or more, or a two-hour value in an oral glucose tolerance test at or above 200 mg/dL.
Dr. Jack Merendino, MD
Endocrinology Diabetes & Metabolism
These are the tests your doctor uses for diagnosing diabetes:
 
  • Fasting blood sugar test. This blood test is done after you have
           fasted for eight or more hours (usually overnight). Normal
           fasting blood sugar is under 100 mg/dL. (This is shorthand for
           milligrams per deciliter.) If your blood glucose is between 100
           and 125 then you meet the criteria for prediabetes; a
           glucose level of 126 or more meets the criteria for diabetes.
  • Non-fasting blood sugar test. If your blood glucose level at any
           time exceeds 200, then you would also be diagnosed with
           diabetes. (There is no standard to indicate prediabetes.)
  • Oral glucose tolerance test (OGTT). For this test, you go to the
           doctor’s office and down a syrupy drink containing 75 grams
           of glucose. Normal blood sugar two hours later is under 140;
           between 140 and 199 qualifies as prediabetes; and if your
           blood sugar tops 200, you’ve tested positive for diabetes.
           This test is usually done when the doctor suspects that you
           have diabetes or prediabetes but standard blood sugar tests
           don’t confirm the diagnosis.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.