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Diagnosing Diabetes

Making a Diagnosis Using the Fasting Plasma Glucose Test

The fasting plasma glucose test is the preferred test for diagnosing the condition, due to its convenience. It is most reliable when done in the morning. Results and their meaning are shown in Table 1. If your fasting glucose level is 100 to 125 mg/dL, you have a form of pre-diabetes called impaired fasting glucose (IFG), meaning that you are more likely to develop type 2 diabetes but do not have it yet. A level of 126 mg/dL or above, confirmed by repeating the test on another day, means that you have diabetes.
 
Table 1: Fasting Plasma Glucose Test
 
Plasma Glucose Result (mg/dL)
Diagnosis
99 and below
Normal
100 to 125
Pre-diabetes
(impaired fasting glucose)
126 and above
Diabetes*
*Confirmed by repeating the test on a different day
 

Diagnosing Diabetes Using the Oral Glucose Tolerance Test

Diabetes research has shown that the oral glucose tolerance test is more sensitive than the fasting plasma glucose test for making a diagnosis, but it is less convenient to administer. The oral glucose tolerance test requires you to fast for at least eight hours before the test. Your plasma glucose is measured immediately before and two hours after you drink a liquid containing 75 grams of glucose dissolved in water. Results and what they mean are shown in Table 2.
 
If your blood glucose level is between 140 and 199 mg/dL two hours after drinking the liquid, you have a form of pre-diabetes called impaired glucose tolerance or IGT, meaning that you are more likely to develop type 2 diabetes but do not have it yet. A two-hour glucose level of 200 mg/dL or above, confirmed by repeating the test on another day, means that you have diabetes.
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