Dr. Aruna D. Pradhan, of Brigham and Women's Hospital, Boston, writes in a current journal that A1c (HbA1c) is a marker of cumulative glycemic exposure over the preceding 60 to 90 days.

The investigators continue to stat that, "Whether mild elevations of this biomarker provide prognostic information for development of clinically evident type 2 diabetes and cardiovascular disease among individuals at usual risk for these disorders is uncertain."

The researchers conducted a prospective cohort study, beginning in 1992,  where they examined whether baseline HbA1c can predict diabetes and a first cardiovascular event in healthy middle-aged and older women.
Included in the analysis were 26,563 participants of the Women's Health Study who were at least 45 years of age and did not have diabetes or cardiovascular disease. The subjects were followed for a median of 10.1 years. At baseline HbA1c levels were 5%.

A total of 1238 cases of diabetes and 684 cardiovascular events occurred during follow-up. In age-adjusted analyses using quintiles of HbA1c, a graded risk increase was observed for both incident diabetes and cardiovascular disease. HbA1c remained a strong predictor of diabetes after multivariable adjustment. However, it was no long significantly associated with incident cardiovascular disease.

The authors repeated analyses according to clinically expedient cutpoints in 0.5% increments above 5.0% to examine threshold effects. The adjusted relative risks for incident diabetes ranged from 2.9 for HbA1c levels between 5.5% and 5.9%; to 29.3 for HbA1c levels between 6.0% and 6.4%; to 81.2 for HbA1c levels of 7.0% or higher.
Risk associations persisted after exclusion of cases diagnosed within 2 and 5 years of follow-up.

From the results, Dr. Pradhan's team concluded that,  “Although these data do not support the use of HbA1c as a single measure of diabetes risk, our results do suggest that the significance of elevated HbA1c may warrant a greater emphasis in primary prevention.".
Am J Med 2007;120:720-727.

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