Strains of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, faecal streptococci, Proteus spp, and Klebsiella spp were distributed on two occasions to two groups of laboratories, one using a commercially produced break point method (Adatab, Mast Laboratories Ltd) and the other using a disc method for susceptibility testing. Minimum inhibitory concentrations of a range of antibiotics were determined for each of the strains in the Division of Microbiological Reagents and Quality Control and a correct result of sensitive or resistant was assigned where possible to each combination of strain and antibiotic. Laboratories were asked to determine the susceptibility of the strains to those antibiotics that they would test in routine practice. Results from each laboratory were compared with the correct results. The overall error rates obtained with the Adatab and disc methods, 8% and 8.2% respectively, were not significantly different. Fewer errors were made with trimethoprim, ticarcillin, and nitrofurantoin by laboratories using Adatabs than those using discs. Fewer errors were made with gentamicin by laboratories using discs than those using Adatabs. There was no significant difference between the two groups of laboratories in reproducibility of results on repeated testing of the same strains. Laboratories using Adatabs used a wide range of different break point concentrations. The Adatab method appeared to offer no overall advantages in terms of reduced error rates or increased reproducibility of results with the strains tested.
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