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LETTER TO THE EDITOR

Nonignorable Covariates

Department of Anesthesiology; University of Utah; Salt Lake City, UT; n.l.pace{at}utah.edu

To the Editor:

Based upon the analysis of observational data from an established surgical registry that included outcomes before 2001 to 2004 and after 2005, Le Manach et al.1 proposed a policy change in the rapidity of the resumption of oral statin therapy to reduce the risk of myocardial ischemic events. Not being a randomized controlled trial, the authors used propensity scores to stratify patients into quartiles for balancing the observed covariates such as patient characteristics or concomitant disease. Propensity scores are a relatively recent statistical technique for reducing the bias of inferences from the use of observational data.2 However, propensity scores may not balance unobserved covariates.

Le Manach et al.1 reported postoperative cardiac myonecrosis in control patients who were not receiving chronic statin therapy (Table 1). From the earlier to the later period, the risk increased [2001–2004, 8% (22/289); 2005 17% (15/86)]; this is statistically significant (Fisher's exact test, P = 0.01). This suggests that other changes, unrelated to statin therapy resumption, occurred between 2001–2004 and 2005 in patient selection, surgical care, medical management, or anesthetic techniques. Because these unobserved or unobservable covariates were not included in the propensity scores, the inference that the rapid resumption of statin therapy is beneficial may be biased.

REFERENCES

1. Le Manach YL, Godet G, Coriat P, Martinon C, Bertrand M, Fléron M-H, Riou B. The impact of postoperative discontinuation or continuation of chronic statin therapy on cardiac outcome after major vascular surgery. Anesth Analg 2007;104:1326–1333[Abstract/Free Full Text]
2. Joffe MM, Rosenbaum PR. Invited commentary: propensity scores. Am J Epidemiol 1999;150:327–333[Abstract/Free Full Text]

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