Laboratory Tests for Diabetes

Laboratory Tests for Diabetes

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.

Hemoglobin A1c test

The HbA1c test measures the amount of glucose attached to hemoglobin—the oxygen-carrying protein in red blood cells that gives blood its color. The HbA1c test is used in people already diagnosed with diabetes and is not recommended for diagnosing diabetes. 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 HbA1c test measures average blood glucose levels over the previous two to three months.

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The American Diabetes Association recommends keeping your HbA1c levels at less than 7%, which is equivalent to an average blood glucose level of about 170 mg/dL or less. Your doctor may give you a different level to aim for, depending on your age, weight, and other factors. HbA1c tests are usually performed every three months to see if you are maintaining your blood glucose within the target range. If you have stable blood glucose levels and are meeting your treatment goals, you may need less frequent HbA1c testing.

Other laboratory tests

In addition to measures of blood glucose and HbA1c, initial and subsequent doctor visits may include tests to check for kidney damage, a common complication of diabetes. These tests include blood urea nitrogen (BUN), blood creatinine, and protein (albumin) in the urine. The risk of coronary heart disease is increased in people with diabetes, so you'lll also need blood tests to measure levels of triglycerides, total cholesterol, low-density lipoprotein (LDL) cholesterol, and high-density lipoprotein (HDL) cholesterol.

Diabetes (Diabetes Mellitus), Laboratory Tests for Diabetes reprinted with permission from
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