In vitro testing of antimicrobial agents.

Indications for determining the inhibitory activity of an antimicrobial agent are well defined, and the data provided by such testing are useful for guiding therapy in a cost-effective manner. Susceptibility test methods with inhibition as an endpoint are standardized, and guidelines regarding methodology, quality control, and reporting are published. The ultimate decision concerning reporting, however, should be made jointly by clinical microbiologists, members of the hospital Pharmacy and Therapeutics or Infection Control Committee, and infectious disease clinicians. The NCCLS also has published tentative guidelines regarding methods for determining bactericidal activity of antimicrobial agents. Given the many biologic and technical variables that influence these tests, standardization is critical for appropriate interpretation of results. In contrast to tests that measure inhibition, however, the clinical value of bactericidal testing is poorly defined, and additional information gained by conducting well-designed, prospective, blinded clinical trials is needed. Methods for determining synergism/antagonism have not been standardized, which makes interpreting results and comparing results of tests performed at different institutions difficult. With the exception of detecting high-level aminoglycoside resistance in enterococci as a means to predict synergism between these drugs and cell wall-active agents, the clinical relevance of assessing synergism is not well documented.