The Path of More Resistance: A Comparison of NHSN and CLSI Criteria in Developing Cumulative Antimicrobial Susceptibility Test Reports and Institutional Antibiograms.

Background In the absence of antimicrobial susceptibility data, the institutional antibiogram is a valuable tool to guide clinicians in the empiric treatment of infections. However, there is a misunderstanding on how best to prepare cumulative antimicrobial susceptibility testing reports (CASTRs) to guide empiric therapy (e.g., routine antibiogram) versus monitoring antimicrobial resistance, with the former following guidance from the Clinical Laboratory Standards Institute (CLSI), and the latter from Center for Disease Control and Preventions National Healthcare Safety Network (NHSN). These criteria vary markedly in their exclusion or inclusion of isolates cultured repeatedly from the same patient. Methods We compared rates of non-susceptibility (NS)using annual data from a large teaching healthcare system subset to isolates eligible by either NHSN criteria or CLSI criteria. Results For a panel of the three most prevalent gram-negative pathogens in combination with clinically relevant antimicrobial agents (or priority pathogen-agent combinations, PPACs), we found that the inclusion of duplicate isolates by NHSN criteria yielded higher NS rates than when CLSI criteria (for which duplicate isolates are not included) were applied. Conclusions Patients with duplicate isolates may not be representative of antimicrobial resistance within a population. For this reason, users of CASTR data should carefully consider that the criteria used to generate these reports can impact resulting NS rates, and therefore maintain the distinction between CASTRs created for different purposes.