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Circulating endothelial progenitor cells.

to the editor: Recent studies have shown that endothelial progenitor cells are mobilized within a few hours after acute myocardial infarction, and the level of these cells in peripheral blood remains elevated for up to several days or weeks in patients with acute myocardial infarction, as compared with patients with stable angina and healthy control subjects.1,2 Among 507 participants in the study by Werner et al. (Sept. 8 issue)3 who had coronary artery disease on angiography, a history of acute, subacute, or myocardial infarction was present in 3.4 percent, 8.5 percent, and 32.1 percent, respectively. The level of endothelial progenitor cells in these patients was elevated to various degrees as a result of previous myocardial infarction. Therefore, categorizing patients according to the level of endothelial progenitor cells without considering the history of myocardial infarction may have caused a misclassification in this study. It is possible that patients who had recently had a myocardial infarction (and therefore were at higher risk for future infarction and death) were wrongly assigned to the group of patients with a medium level or high level of endothelial progenitor cells among those who had never had a myocardial infarction. This potential misclassification may explain the lack of the association between the level of endothelial progenitor cells and the incidence of myocardial infarction and death from all causes or acute myocardial infarction.