Four blood tests are available to diagnose prediabetes and diabetes:
• Fasting plasma glucose (FPG)
• Oral glucose tolerance test (OGTT)
• Hemoglobin A1c (A1c) test
• Random plasma (blood) glucose
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 A1c tests to monitor your blood glucose control.
1. 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 to 125 mg/dL indicate prediabetes. A normal fasting blood glucose level is less than 100 mg/dL.
2. Oral glucose tolerance test (OGTT)
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 blood glucose level at two hours is 140 to 199 mg/dL.
3. Hemoglobin A1c (A1c) test
This blood test measures the amount of glucose attached to hemoglobin—the oxygen-carrying protein in red blood cells that gives blood its color. The A1c test was originally used to monitor glucose levels in people already diagnosed with diabetes, but it is now also used to diagnose type 2 diabetes and prediabetes. (The test is not recommended for diagnosing diabetes in people with certain conditions with abnormal red blood cell turnover that could affect the test results; this includes pregnancy, some anemias, and recent blood loss or transfusion.)
As blood glucose levels rise, so does the amount of glucose attached to hemoglobin. Since hemoglobin circulates in the blood until the red blood cells die (half of red blood cells are replaced every 120 days), the A1c test measures average blood glucose levels over the previous two to three months. You do not have to fast to have the A1c test.
An A1c level of 6.5 percent or higher meets American Diabetes Association (ADA) criteria for the diagnosis of diabetes. In general, the ADA recommends keeping your A1c levels at or below 7 percent, which is equivalent to an average blood glucose level of about 154 mg/dL or less. Your doctor may tell you to aim for a different target, depending on your age, treatment, and other factors. A category of high risk for diabetes (similar to prediabetes) is diagnosed based on A1c levels of 5.7 to 6.4 percent.
A1c tests are usually performed every three months in people with diabetes to see if blood glucose levels are within the target range. If you have stable blood glucose levels and are meeting your treatment goals, you may need less frequent A1c testing.
4. Random plasma (blood) glucose test
This test measures blood glucose levels at any time of day, no matter when you had your last meal; however, the ADA does not recognize it as a preferred diagnostic test. 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 are the presence of diabetes symptoms and a blood glucose level of 200 mg/dL or higher, although, in our opinion, if a random plasma glucose is in the 140 to 200 mg/dL range, you should be checked with a fasting plasma glucose test.
Other laboratory tests
In addition to measures of blood glucose and A1c, initial and subsequent doctor visits may include tests to check for kidney damage, which is a complication that is common in individuals with diabetes. Among the tests the doctor will order are a urinalysis to determine if you have protein in the urine (microalbumin). If protein is present, your doctor can follow up with blood tests: blood urea nitrogen (BUN) and blood creatinine.
The risk of coronary heart disease is increased in people with diabetes. Consequently, the doctor will also need to order blood tests to measure levels of triglycerides, total cholesterol, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol.