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To the Editor: Dr Malenka and colleagues¹ described the protective effect of drug-eluting stents against repeat revascularization procedures by comparing frequency of revascularization in the bare-metal stent era to the frequency after drug-eluting stents came on the market. In doing so, they took a step back from the exposure itself—drug-eluting vs bare-metal stent—and instead implicitly used the era as a proxy for the stent exposure. Presumably, they did this because they believed that the exposure to drug-eluting stent would be confounded by unmeasured factors that they could not adjust for with the available data.

A factor that is related to exposure but does not confound the outcome, via either measured or unmeasured paths, is an instrumental variable.² From the study by Malenka et al, stenting era may well be an instrumental variable for the confounded exposure of stent type, particularly if underlying trends in care that may have affected the . . . [Full Text of this Article]

Jeremy A. Rassen, ScD
jrassen@post.harvard.edu

Sebastian Schneeweiss, MD, ScD
Division of Pharmacoepidemiology and Pharmacoeconomics
Harvard Medical School
Boston, Massachusetts

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