March 8, 2010

Hemoglobin A1c outperforms fasting glucose for risk prediction

Measurements of hemoglobin A1c more accurately identify persons at risk for clinical outcomes than the commonly used measurement of fasting glucose, according to a study by researchers at the Johns Hopkins Bloomberg School of Public Health.

Levels of hemoglobin A1c, or HbA1c, accurately predict future diabetes, and they better predict stroke, heart disease and all-cause mortality as well. The study appears in the March 4 issue of The New England Journal of Medicine.

As a diagnostic, “HbA1c has significant advantages over fasting glucose,” said Elizabeth Selvin, the study’s lead author. The A1c test has low variability from day to day, levels are not as affected by stress and illness, it has greater stability, and the patient is not required to fast before the test is performed.

This study is published on the heels of a major change in the way doctors diagnose diabetes. In January, the American Diabetes Association published revised recommendations for the screening and diagnosis of diabetes. The revised recommendations include, for the first time, recommendations to use HbA1c to diagnose diabetes and also to identify people at risk of developing diabetes in the future.

The new findings can help doctors and patients interpret HbA1c test results.

In the study, people with HbA1c levels between 5.0 percent to 5.5 percent were identified as being within normal range. The majority of the U.S. adult population is within this range. With each incremental HbA1c increase, the incidence of diabetes increases as well, the study found; those at a level of 6.5 percent or greater are considered diabetic, and those between 6.0 and 6.5 percent are considered at a very high risk (nine times greater than those at the normal range) for developing diabetes. The revised ADA guidelines classify people with HbA1c levels in the range of 5.7 to 6.4 percent as at very high risk for developing diabetes over five years. The range of 5.5 percent to 6 percent, according to the ADA guidelines, is the appropriate level to initiate preventive measures.

The study measured HbA1c in blood samples from more than 11,000 people, black and white adults, who had no history of diabetes. The samples were obtained between 1990 and 1992 as part of the Atherosclerosis Risk in Communities Study at four ARIC field centers in Hagerstown, Md.; Minneapolis; Jackson, Miss.; and Forsyth County, N.C. The samples have been in cold storage since their collection, and researchers were able to look at participants’ measurements and compare them to outcomes that occurred during 15 years of follow-up. “It is amazing to be able to use blood samples collected over a decade ago,” Selvin said.

Currently there are 9 million Americans who are diabetic but undiagnosed. “These data,” Selvin said, “can help us interpret A1c values in clinical practice and help identify people who need treatment the most.”

This research was supported by NIH/NIDDK grants and the Johns Hopkins Diabetes Research and Training Center. The Atherosclerosis Risk in Communities Study is carried out as a collaborative study supported by the National Heart, Lung, and Blood Institute. In addition to Selvin, study authors from Johns Hopkins are Hong Zhu, Kunihiro Matsushita, Josef Coresh and Frederick L. Brancati.