Normalized resistance interpretation as a tool for establishing epidemiological MIC susceptibility breakpoints

Normalized resistance interpretation (NRI) utilizes the fact that the wild-type population on the sensitive side is not affected by resistance development, and therefore a normalized reconstruction of the peak can be performed. The method was modified for MIC distributions by the introduction of helper variables, in-between values assigned the mean of the neighboring numbers of isolates. This method was used on Staphylo- coccus aureus and Escherichia coli MIC distributions for 27 antimicrobials each and obtained from the EUCAST (European Committee on Antimicrobial Susceptibility Testing) website (http://www.eucast.org/mic_distributions/). The number of isolates in each of the 54 distributions ranged from 40 to 124,472. NRI produced normalized distributions in all cases. Cutoff values were calculated for +2.0 and +2.5 standard deviations (SD) above the means and then rounded up to nearest regular MIC dilution step. EUCAST also show cutoff values, ECOFF values, which were used as the reference. The NRI generated +2.0 SD values showed the best agreement with 26 of 27 within ±1 dilution step and 17 exactly on the ECOFF values for Staphylococcus aureus, and 25 of 27 within ±1 dilution step and 14 right on the ECOFF values for Escherichia coli. NRI offers an objective method for the reconstruction of the wild-type population in an MIC distribution for a given bacterial species and an antimicrobial agent. This method offers a new tool in comparative susceptibility studies such as global surveillance of resistance, as well as in quality control in individual laboratories.