Seven laboratories participated in a collaborative study to

A multilaboratory in vitro study was carried out to determine disk diffusion susceptibility testing quality control limits for two new semisynthetic penicillins, mezlocillin and piperacillin. Existing limits for carbenicillin and ampicillin were reevaluated. Multiple tests (which followed standards set by the National Committee for Clinical Laboratory Standards, ASM-2 revised) were performed in nine laboratories by different technologists using disks and Mueller-Hinton agar from different manufacturers. Clinically significant differences between disks produced by different manufacturers were not noted. Inhibitory zone diameter measurements from all laboratories were analyzed, and upper and lower control limits were established by using the overall median +0.5 the median range of the individual laboratory measurements as determining parameters. Close agreement of the data in this study with the results of national proficiency testing and quality control programs for ampicillin and carbenicilhin supports the validity of our approach to making initial recommendations for quality control guidelines for new antimicrobial agents.

This three-center collaborative study was conducted to evaluate samples of Sensititre antimicrobial microdilution panels (GIBCO/INVENEX). Sensititre miniimum inhibitory concentrations of 27 bacterial isolates were compared with those obtained by a reference microdilution method. The Sensititre and microdilution miniimum inhibitory concentrations were equivalent within ±1 dilution in 87.6% of the comparable test results. Three separate Sensititre antimicrobial panels were used in this study. Table 2 lists the antimicrobials and the range of drug concentrations tested for each of these panels. Currently available panels may vary somewhat from the prototypes used in this study.Reference microdilution panels duplicated the antimicrobial and drug concentration range of the Sensi-464

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Micro-Media Systems (MMS) has developed a procedure by which microdilution trays can be filled with dilutions of antimicrobial agents, frozen, and distributed to clinical laboratories. The trays are prepared in various distribution centers throughout the United States to supply clinical laboratories in the vicinity of each center. For use, trays are removed from the freezer, allowed to thaw, and then inoculated with inocula prepared as for any other susceptibility test, using a convenient disposable inoculator (ca. 5 ,ul per well). A collaborative study was planned to evaluate microdilution trays prepared in three MicroMedia Systems distribution centers. Microdilution minimal inhibitory concentrations (MICs) were compared to standard tube dilution tests (the international collaborative study group method). With gram-positive cocci, the two techniques gave essentially equivalent results. With gram-negative bacilli, the microdilution MICs were generally one doubling dilution lower than the standard tube dilution MICs. Similar results were seen with microdilution trays prepared with a Cooke Dynatech MIC 2000. Inter-and intralaboratory reproducibility with the macro-and microdilution techniques were quite satisfactory, i.e., at least 96% of the end points were within a range of + 1 log2 dilution intervals.