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Diagnosis of Diabetes

How Are Diabetes and Prediabetes Diagnosed?

The following tests are used for the diagnosis of diabetes:

  • A fasting plasma glucose test measures your blood glucose after you have gone at least 8 hours without eating. This test is used to detect diabetes or prediabetes.
  • An oral glucose tolerance test measures your blood sugar after you have gone at least eight hours without eating and two hours after you drink a glucose-containing beverage. This test can be used to diagnose diabetes or prediabetes.
  • In a random plasma glucose test, your doctor checks your blood sugar without regard to when you ate your last meal. This test, along with an assessment of symptoms, is used to diagnose diabetes, but not prediabetes.

Positive test results should be confirmed by repeating the fasting plasma glucose test or the oral glucose tolerance test on a different day. When first diagnosed with diabetes, your doctor may suggest a zinc transporter 8 autoantibody (ZnT8Ab) test. This blood test -- along with other information and test results -- can help determine if a person has type 1 diabetes and not another type. The goal of having the ZnT8Ab test is a prompt and accurate diagnosis and that can lead to timely treatment.

Fasting Plasma Glucose (FPG) Test

The FPG is the preferred test for diagnosing diabetes and 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 prediabetes 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 Prediabetes
(impaired fasting glucose)
126 and above Diabetes*

*Confirmed by repeating the test on a different day.

Oral Glucose Tolerance Test (OGTT)

Research has shown that the OGTT is more sensitive than the FPG test for diagnosing prediabetes, but it is less convenient to administer. The OGTT 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 sugar level is between 140 and 199 mg/dL 2 hours after drinking the liquid, you have a form of prediabetes 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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