Consensus Moving to A1c as a Better Predictor of Diabetes Than Fasting Glucose
Over the past few months, there has been a discernible shift of opinion among healthcare providers about which test best reveals a high risk of acquiring diabetes. The old standby, fasting glucose, seems to be giving way to the hemoglobin A1c test as the preferred method.
A just-published study by researchers at the Johns Hopkins Bloomberg School of Public Health contends not only that the A1c better identifies persons at risk for diabetes than fasting glucose, but also that it can better predict stroke, heart disease, and overall mortality.
To prepare for a fasting glucose test, the patient must fast first, usually from midnight until the time a blood sample is drawn the next day. The level of blood glucose in the sample gives a good indication of whether the patient is non-diabetic, is at risk for diabetes, or has the disease. An A1c, on the other hand, indicates blood glucose levels over an extended period, usually 90 days, and does not require patients to fast.
Among the advantages of an A1c over the fasting glucose test, said the study's lead author, Elizabeth Selvin, PhD, MPH, is its low variability and reduced susceptibility to variance based upon illness or stress. For that reason, it is much less likely to produce a "false alarm" that could lead to a misdiagnosis.
Another advantage is that A1c levels are rendered in percentages that are easy to understand. The Johns Hopkins study identified an A1c of 5.0% to 5.5% as within the normal range. As the A1c percentage increases, so does the risk of acquiring diabetes. A1c's of 5.5% to 6% are considered "very high risk," indicating a nine-times greater likelihood of getting diabetes than an A1c within the normal range.
A patient with an A1c of 6.5% or greater, according to the researchers, should be considered to have diabetes. (The American Diabetes Association guidelines say that anyone with an A1c from 5.7% to 6.4% is "at very high risk" of acquiring diabetes within five years, while a range of 5.5% to 6% indicates a need to take preventive measures.)
The study relied on blood samples taken in the early 1990s from 11,000 black and white adults who did not have diabetes. By measuring the A1c's in those samples and then tracking which participants eventually developed diabetes, the researchers were able to establish some pretty solid indicators of diabetes risk.
The Johns Hopkins data are one more push in the direction of establishing the A1c as the preferred test for diabetes risk. Look for doctors and HMOs to begin offering it as a standard component in routine annual check-ups.
The study appears in the March 4, 2010, issue of New England Journal of Medicine.
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