Susceptibility testing today: myth, reality, and new direction

In vitro susceptibility testing versus in vivo effectiveness

The clinical relevance of susceptibility testing has always been questioned because of the difficulty of correlating in vitro susceptibility testing with in vivo clinical effectiveness. Clearly there have always been host/pathogen factors that influence the clinical outcome that cannot be predicted by the results of susceptibility testing. However, improved understanding of pharmacodynamic and pharmacokinetic parameters has greatly improved the use of antimicrobial agents. Most importantly, the integration of these pharmacodynamic and pharmacokinetic indices has greatly improved the correlation between in vitro susceptibility testing and in vivo clinical effectiveness and allows more realistic breakpoints. Finally, the clinical microbiology laboratory has advanced with improved methods as well as the adaptation of breakpoints that are more realistic.

The problem of selecting antimicrobial agents before (or without) the results of susceptibility testing

The appropriate selection of an antimicrobial regimen depends upon more factors than merely the results of susceptibility testing. It is a task more difficult today because of the recent development of many new antimicrobial agents. Clinicians who prescribe antibiotics need to invest time and energy beyond reading a package insert in order to gain familiarity with antimicrobial agents. Understanding the antimicrobial and pharmacologic properties of a few agents from each class of antimicrobial agents will help make the selection of empiric regimens less difficult. Essential to empiric therapy is frequent and habitual reassessment of antimicrobial regimens with possible application of cost-effective modifications kept in mind. Once the pathogen is isolated, the empiric regimen can be modified. The very last factor that is used in modification of the antimicrobial regimen is the result of susceptibility testing. Prior to (or without) this information, the clinical outcome of serious infections most often will depend upon the early institution of appropriate empiric therapy.