The development of the BSAC standardized method of disc diffusion testing

The BSAC Working Party on Susceptibility Testing has developed a standardiz ed method of disc susceptibility testing that has been 'field tested' in 19 diagnostic laboratories in the UK and Ireland. The method employs semi-def ined media, a semi-confluent inoculum and relates zones of inhibition with BSAC-specified MIC breakpoints to interpret susceptibility. The bacteria se lected for the trial included clinical isolates and control strains from th e ATCC and NCTC national collections. Organisms were chosen because they ha d known attributes, such as being fully susceptible or having a demonstrate d mechanism of resistance. The results from this survey are very encouragin g. With the commonly isolated Enterobacteriaceae, specifically Escherichia coli, Proteus mirabilis and Klebsielia spp., no major problems were observe d except with gentamicin and cefuroxime. In the case of gentamicin, problem s were associated with resistant strains of P. mirabilis, with MICs of 2 mg /L, being falsely reported as susceptible. For cefuroxime, it is not unexpe cted that results were unreliable, as the MIC distribution straddles the in vitro breakpoint concentration (following the results of this study the MI C and zone diameter breakpoints have been amended to improve reporting). No major problems were encountered for Pseudomonas aeruginosa with the agents studied. The 'field survey' has shown that disc testing is unreliable for determining the susceptibility of coagulase-negative staphylococci to telco planin, and that the detection of glycopeptide resistance in enterococci is improved by incubation for a full 24 h. Inconsistencies observed with fast idious organisms were associated with incorrect inocula. Zone diameter data for the control strains studied provide information that can be utilized b y diagnostic laboratories to monitor the daily performance of testing.