DEVELOPMENT OF INTERPRETIVE CRITERIA AND QUALITY-CONTROL LIMITS FOR BROTH MICRODILUTION AND DISK DIFFUSION ANTIMICROBIAL SUSCEPTIBILITY TESTING OF STREPTOCOCCUS-PNEUMONIAE

A five-center collaborative study was undertaken to develop quality co ntrol and specific interpretive criteria for susceptibility testing of Streptococcus pneumoniae against 12 antimicrobial agents. MICs were d etermined for 248 pneumococcal clinical isolates (with an emphasis on resistant strains) by use of the National Committee for Clinical Labor atory Standards (NCCLS)-recommended broth microdilution procedure inco rporating lysed horse blood-supplemented Mueller-Hinton broth. NCCLS d isk diffusion testing was also performed for each isolate by using Mue ller-Hinton sheep blood agar incubated in 5% CO2. Repetitive testing o f S. pneumoniae ATCC 49619 with different sources and lots of media an d disks allowed development of quality control ranges which encompasse d approximately 95% of MIC and zone size values observed in the study. Good intra- and interlaboratory reproducibilities were seen with thes e testing methods and all of the drugs examined. On the basis of the r esults of this study, MIC interpretive criteria are proposed for II ag ents. Comparisons of MICs and disk diffusion zone sizes allowed disk d iffusion zone size interpretive criteria to be proposed for five drugs and confirmed the use of the oxacillin disk test for prediction of pe nicillin susceptibility among pneumococci. Excessive numbers of minor- category interpretive errors precludes recommendation at this time of the disk diffusion method for testing of pneumococci against five of t he drugs. Use of these proposed quality control and interpretive crite ria should provide for reproducible test results and allow recognition of recently emerging resistance among pneumococcal clinical isolates.