From the instant antimicrobial agents began to be wont to treat patients suspected of getting infectious diseases it had been obvious that there was a requirement to predict efficacy. Clearly, in vitro susceptibility tests were required and initial testing was based upon determining the MIC, which was first developed by Fleming and is well described in his 1929 paper.1 Since then there has been considerable work to develop practical, reliable and reproducible laboratory methods for susceptibility testing, methods to correlate in vitro laboratory tests to clinical outcome, and methods to spot new resistance mechanisms of clinical importance.
In 1960, an expert committee of the WHO met in Geneva and recommended the definition of ordinary routine methods for antibiotic susceptibility testing.2 This led to a world collaborative study sponsored by the WHO that examined most aspects of susceptibility testing over subsequent 10 years and eventually published recommendations in 1971.3 Although the first WHO committee of 1960 had intended the event of one , universally adopted method, the 1971 report suggested that there should be national reference laboratories with the duty of ‘preparing complete descriptions of current recommended methods and interpretative schemes’.Serial dilution methods were labour intensive and time consuming and a way replacing serial dilutions with one or more critical concentrations that separated organisms into resistant and susceptible categories was devised: the ‘breakpoint’ technique, a term first employed by Ericsson and Sherris.3 the primary disc diffusion method was developed within the 1950s to exchange the MIC.4 The zone diameters of disc diffusion tests were correlated to MICs extensively during this decade to consolidate their usefulness within the laboratory.5,6 The report by Ericsson and Sherris (1971) described a world collaborative study that established the idea for standardized susceptibility testing methods, especially disc diffusion tests, and also explained the connection between MIC and zone diameter.3 By 1972 the National Committee for Clinical Laboratory Standards (NCCLS) [now the Clinical Laboratory Standards Institute (CLSI)] had published a tentative standard that recommended the Kirby–Bauer disc diffusion method.7 like any new technique, variations in how the disc diffusion was performed abounded and therefore the effect of those variations on testing phenomena required explanation.