Diagnosing Diabetes with the HbA1c Blood Test

Type 2 diabetes usually develops gradually over many years and the initial symptoms may be almost unnoticeable. In fact, many people find out that they have type 2 diabetes when a routine laboratory test shows high blood glucose levels. Increasingly endocrinologists are using the hemoglobin A1c (HbA1c) test -- which is now used to monitor glucose control in existing patients -- to diagnose diabetes.

If you have any of the common symptoms that suggest the presence of diabetes, your doctor can order a blood glucose test to confirm or rule out the diagnosis. Three blood tests commonly used to diagnose prediabetes and diabetes:

• casual plasma (blood) glucose
• fasting plasma glucose (FPG)
• oral glucose tolerance test (OGTT)

About six million people with diabetes in the United States don't know they have diabetes, and up to one quarter of U.S. people with diabetes have evidence of serious complications by the time they are diagnosed. That's why a panel of experts -- including Dr. Christopher D. Saudek, the author of The Johns Hopkins Diabetes White Paper -- convened to consider whether the HbA1c test would be a more effective tool for screening for and diagnosing diabetes. The HbA1c test measures the amount of glucose attached to hemoglobin. The test is routinely used to assess blood glucose control over the previous two to three months.

The current tests used for the diagnosis of diabetes -- FPG and OGTT -- have flaws. They must be administered when a person has fasted for at least eight hours, which is often a deterrent. And their results may not be entirely accurate, improving with short-term lifestyle changes such as when a person eats healthier and exercises in the days prior to the test.

An HbA1c test, by contrast, can be administered at any time and offers a long-term view of an individual's glucose control. With these and other factors in mind, the panel recommended using an HbA1c cutoff of 6.5% as a screening test for diabetes and suggested follow-up tests to confirm the diagnosis of diabetes. These recommendations await consideration by major medical associations, but one day they may allow many people to become diagnosed sooner. Reported in the Journal of Clinical Endocrinology and Metabolism (Volume 93, page 2447).