Antimicrobial Proficiency Testing of National Nosocomial Infections Surveillance System Hospital Laboratories

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Abstract
Objective: : The National Nosocomial Infections Surveillance (NNIS) System personnel report trends in antimicrobial-resistant pathogens. To validate select antimicrobial susceptibility testing results and to identify test methods that tend to produce errors, we conducted proficiency testing among NNIS System hospital laboratories.Setting: : NNIS System hospital laboratories in the United States.Methods: : Each laboratory received five organisms (ie, an imipenem-resistantSerratia marcescens,an oxacillin-resistantStaphylococcus aureus,a vancomycin-resistantEnterococcus faecalis,a vancomycin-intermediateStaphylococcus epidermidis,and an extended-spectrum beta-lactamase (ESβL)-producingKlebsiella pneumoniae).Testing results were compared with reference testing results from the Centers for Disease Control and Prevention.Results: : Of 138 laboratories testing imipenem against theSerratia marcescensstrain, 110 (80%) correctly reported minimum inhibitory concentrations (MICs) or zone sizes in the resistant range. All 193 participating laboratories correctly reported theStaphylococcus aureusstrain as oxacillin resistant. Of the 193 laboratories, 169 (88%) reported correct MICs or zone sizes for the vancomycin-resistantEnterococcus faecalis.One hundred sixty-two (84%) of 193 laboratories demonstrated the ability to detect a vancomycin-intermediate strain ofStaphylococcus epidermidis;however, disk diffusion performed poorly when testing both staphylococci and enterococci with vancomycin. Although laboratory personnel correctly reported nonsusceptible extended-spectrum cephalosporins and aztreonam results forK. pneumoniae,only 98 (51%) of 193 correctly reported this organism as an ESβL producer.Conclusion: : Overall, NNIS System hospital laboratory personnel detected most emerging resistance patterns. Disk diffusion continues to be unreliable for vancomycin testing of staphylococci and must be used cautiously for enterococci. Further education on the processing of ESβL-producing organisms is warranted.

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