European standards for antibiotic susceptibility testing: towards a theoretical consensus

Six different systems for antimicrobial susceptibility testing are used in Europe (BSAC, DIN, SFM, SIR, NCCLS and WRG). Breakpoints defining susceptibility or resistance of bacteria to antimicrobial agents remain the subject of discussion. Nevertheless, it is possible to establish a theoretical consensus standard list of breakpoints such that more than 95% of the breakpoints proposed by the different systems differ from the consensus standard by no more than one dilution. In general, the BSAC and DIN systems recommend lower breakpoints, and the SFM and NCCLS systems higher breakpoints than the consensus standard. Two-thirds of the discrepancies are related to the breakpoints defining susceptibility. The breakpoint for susceptibility should reflect the intrinsic susceptibility of the isolate (absence of mechanisms of resistance), and the breakpoint for resistance the pharmacological potential of the drug. The scientific validation of breakpoints requires clinical verification.