Measurements of haemoglobin A1c (Hb A1C) more accurately identify persons at risk for clinical outcomes than the commonly used measurement of fasting glucose, according to a study published in the March 4 issue of the New England Journal of Medicine.Hb A1C levels accurately predict future diabetes, and they better predict stroke, heart disease, and all-cause mortality as well.
As a diagnostic, "Hb A1C has significant advantages over fasting glucose," said lead author Elizabeth Selvin, PhD, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland. 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 (ADA) published revised recommendations for the screening and diagnosis of diabetes. The revised recommendations include, for the first time, recommendations to use Hb A1C 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 Hb A1C test results. In the study, people with Hb A1C levels between 5.0% to 5.5% 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 study found, the incidence of diabetes increased as well; those at a level of 6.5% or greater are considered diabetic, and those between 6.0% and 6.5% are considered at a "very high risk" (9 times greater than those at the "normal" range) for developing diabetes.
The revised ADA guidelines classify people with Hb A1C levels in the range of 5.7% to 6.4% as "at very high risk" for developing diabetes over 5 years. The range of 5.5% to 6%, according to the ADA guidelines, is the appropriate level to initiate preventive measures.
The study measured Hb A1C 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 (ARIC) Study at 4 ARIC field centres in Hagerstown, Maryland. The samples have been in cold storage since their collection. "It is amazing to be able to use blood samples collected over a decade ago," said Dr. Selvin, because researchers were able to look at participants' measurements and compare to outcomes that occurred during 15 years of follow-up.
