Three blood tests are available to diagnose prediabetes and diabetes:
- Casual plasma (blood) glucose
- Fasting plasma glucose (FPG)
- Oral glucose tolerance test.
To make a diagnosis, the results of each test must be confirmed by repeat testing on a different day, unless you have obvious symptoms of elevated blood glucose (hyperglycemia). If diabetes is diagnosed, you'll need periodic hemoglobin A1c (HbA1c) tests to monitor your blood glucose control.
Casual plasma (blood) glucose test
This test measures blood glucose levels at any time of day, no matter when you had your last meal. It is most often used in people who have classic diabetes symptoms such as excessive thirst, frequent urination, and unexplained weight loss. The criteria for a diagnosis of diabetes with this test is the presence of diabetes symptoms and a blood glucose level of 200 mg/dL or higher.
Fasting plasma glucose (FPG) test
The fasting plasma glucose test is the preferred method for diagnosing diabetes in children, men, and nonpregnant women. The test measures blood glucose levels after an overnight fast (no food intake for at least eight hours). A diagnosis of diabetes is made when the fasting blood glucose level is 126 mg/dL or higher on at least two tests. Values of 100–125 mg/dL indicate prediabetes. A normal fasting blood glucose level is less than 100 mg/dL.
Oral glucose tolerance test
This test is done when diabetes is suspected, but you have normal results on a fasting plasma glucose test. For the test, you'll have to fast overnight and then drink a very sweet solution containing 75 g of glucose. A sample of your blood will be drawn two hours later. Normal glucose levels are less than 140 mg/dL at two hours. The criterion for a diagnosis of diabetes with this test is a two-hour blood glucose level of 200 mg/dL or higher. Prediabetes is diagnosed if the two-hour blood glucose level is 140–199 mg/dL.