Ability of laboratories to detect emerging antimicrobial resistance: Proficiency testing and quality control results from the World Health Organization's External Quality Assurance System for Antimicrobial Susceptibility Testing

The accuracy of antimicrobial susceptibility data submitted by microbiology laboratories to national and international surveillance systems has been d ebated for a number of years. To assess the accuracy of data submitted to t he World Health Organization by users of the WHONET software, the Centers f or Disease Control and Prevention distributed six bacterial isolates repres enting key antimicrobial-resistance phenotypes to approximately 130 laborat ories, ail but one of which were outside of the United States, for antimicr obial susceptibility testing as part of the World Health Organization's Ext ernal Quality Assurance System for Antimicrobial Susceptibility Testing. Ea ch laboratory also was asked to submit 10 consecutive quality control value s for several key organism-drug combinations. Most laboratories were able t o detect methicillin (oxacillin) resistance in Staphylococcus arureus, high -level vancomycin resistance in Enterococcus faecium, and resistance to ext ended-spectrum cephalosporins in Klebsiella pneumoniae. Many laboratories, particularly those using disk diffusion tests, had difficulty in recognizin g reduced susceptibility to penicillin in an isolate of Streptococcus pneum oniae. The most difficult phenotype for laboratories to detect was reduced susceptibility to vancomycin in an isolate of Staphylococcus epidermidis. T he proficiency testing challenge also included a request for biochemical id entification of a gram-negative bacillus, which most laboratories recognize d as Enterobacter cloacae. Although only a small subset of laboratories hav e submitted their quality control data, it is clear that many of these labo ratories generate disk diffusion results for oxacillin when testing S. aure us ATCC 25923 and S, pneumoniae ATCC 49619 that are outside of the acceptab le quality control range. The narrow quality control range for vancomycin a lso proved to be a challenge for many of the laboratories submitting data; approximately 27% of results were out of range. Thus, it is important to es tablish the proficiency of laboratories submitting data to surveillance sys tems in which the organisms are tested locally, particularly for penicillin resistance in pneumococci and glycopeptide resistance in staphylococci.