W e read with great interest the article in an issue of Antimicrobial Agents and Chemotherapy by Kim et al. (1). The article concluded that the mortality rates of patients stratified by penicillin susceptibility might be relevant to the treatment failures of ampicillin and/or piperacillin in patients with an ampicillin-susceptible but penicillinresistant (ASPR) Enterococcus faecalis bloodstream infection (BSI). Though an interesting and valuable study has been conducted, some methodological

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... ts need to be taken into account. First, the values of crude effect estimates should change Ն10% after adjusting for other covariates in the multivariable model (2, 3). However, as shown in Table 1 in the article (1), for some predictors, the differences between the odds ratio (OR) values in univariate and multivariable models is far lower than the 10% cutoff. For example, the crude OR of cardiovascular disease changed only by 0.35% ([(OR crude -OR adjusted)/OR adjusted ϫ 100%] ϭ [(2.97 -2.98)/2.98 ϫ 100%] ϭ 0.34%); thus, biases might exist in the confounder adjustment. Second, in the study by Kim et al. (1) , variables with a P value of Ͻ0.05 in the univariate analysis were analyzed whether to be included in the multivariate model. Nevertheless, only variables with P Ͻ 0.05 were imported into the multivariate model, which is questionable, as this strategy could lead to a phenomenon called testimation bias, which infers that only variables with a large effect were included and variables with a small effect did not enter into the model. This type of bias can be decreased when independent variables are imported into the multivariate model based on a P value of Ն0.2 (4, 5).