Tracking of glycated hemoglobin in the original cohort of the Framingham Heart Study

Glycated hemoglobin measures average blood glucose over the preceding 2 to 3 months. The authors examined the tracking of the major glycated hemoglobin, hemoglobin A(1c) (HbA(1c)), over a period of 4 to 6 yeats. Two HbA(1c) measurements were obtained between 1986 and 1993 from 639 elderly, presumptively nondiabetic members of the original cohort of the Framingham Heart Study, Framingham, Massachusetts. Mean +/-C standard deviation (SD) baseline and follow-up HbA(1c) were 5.43% +/- 0.7 and 5.71% +/- 0.9, respectively. Intraclass correlation of 0.59 between baseline and follow-up measurements indicated good reliability of a single HbA(1c) measurement. Ninety-one percent of follow-up measurements were within +/- 20% of baseline value; HbA(1c) values tended to move 15% closer to the baseline mean over time. There was a modest tendency for HbA(1c) values to increase with time; the mean difference between measurements was 0.28% +/- 0.7 SD (P < 0.0001), Change in HbA(1c) was positively associated with age and body mass index at baseline examination, and negatively associated with cigarette smoking, even after controlling for age and body mass index. These effects were very small, however. We conclude that glycated hemoglobin reliably categorizes the glucose control of nondiabetic subjects over a period of 4 to 6 years, confirming its value as an epidemilogical measure.