Validation of the NCCLS proposal to use results only from the first isolate of a species per patient in the calculation of susceptibility frequencies

Objective: A recently proposed guideline from the NCCLS recommends that results only from the first isolate of a species per patient be used in calculation of percentage susceptibilities to antimicrobial agents. Because this is apparently based on the comparison of various calculation methods for results for oxacillin against a fairly small number of isolates of Staphylococcus aureus, we have applied these methods to a wider range of antibiotic/organism combinations. Methods: Antibiotic susceptibility results from our hospital laboratory database were analysed. Rates of antimicrobial susceptibility were calculated using the various criteria proposed by the NCCLS, including exclusion of results from duplicate isolates and surveillance specimens from the calculations. Results and conclusion: Analysis of results for methicillin against S. aureus, gentamicin against Klebsiella spp., vancomycin against enterococci (all in-patient specimens), and amoxicillin and cefuroxime against Escherichia coli (general practice specimens) confirm that, if duplicates and surveillance specimens are excluded, results obtained with the various patient- and episode-based methods for the calculation of percentage susceptibility are very similar. Because of its simplicity and unambiguity, we agree with the suggestion of the NCCLS group that results for the 'first isolate of a given species per patient per analysis period, irrespective of body site, antimicrobial susceptibility profile or other phenotypic characteristics' should be used in the calculation of susceptibility frequencies.