The impact of antimicrobial resistance on patient outcomes can be effectively measured only if the appropriateness of the antimicrobial therapy received is properly measured. Definition of appropriate therapy should include not only in vitro susceptibility but also the clinical adequacy of the antibiotic used, taking into account the pathogen isolated, the site of infection, known pharmacokinetic and pharmacodynamic properties of the drug, and dosing. In the absence of these data, the effect of delay or absence of appropriate therapy in patients infected with resistant bacterial pathogens is subject to confounding, and the true effect of resistance on outcomes may be obscured.