A simplified and accurate approach to agar diffusion antibiotic assay

The current linear microbiological assay for antimicrobial susceptibility test (AST) discs is invalid when the calibration line is curved. The alternative of using a multipoint assay is difficult to implement and hence rarely employed. There is therefore a need for a three-point assay method that takes into account non-linear re-lationships. The method described in this article uses data transformation to provide a straight-line calibration thus obviating the need for more difficult multi-point assays. Two examples taken from the literature have been reworked, one a quadratic and the other a cubic fit. In both cases a more accurate assay has been achieved than would otherwise be possible. This method provides a cost-effective and straightforward assay which can increase the quality of AST discs and hence potentially improve clinical outcomes.