Excess Shock and Mortality in Staphylococcus aureus Related to Methicillin Resistance

Abstract

Sir—The study by Soriano et al. [1] provides additional information toward the goal of understanding the impact of antibiotic resistance on patient outcome. The study data are an important contribution to the growing body of literature on this subject; however, there are several points worth making with regard to the stated methodology and results of the study. For example, it should be noted that, in the cohort study, methicillin-resistant Staphylococcus aureus (MRSA) was found to be an independent risk factor for the development of shock. To determine predictors of mortality, the authors then used shock as a predictor in the nonconditional logistic regression analysis, even though their definition of “bacteremia” required the presence of clinically apparent signs and symptoms of sepsis, and even though their definition of “mortality” required that it be related to the episode of bacteremia. If shock is an intermediate step in the direct causal pathway between MRSA and mortality, then it is not a confounder and should not be controlled for in the logistic regression analysis [2]. In this instance, controlling for shock would be similar to controlling for coronary artery disease in a study that looks at the risk of cholesterol in the development of myocardial infarction and then concludes that cholesterol is not a risk factor for myocardial infarction.