Antimicrobial proficiency testing of National Nosocomial Infections Surveillance System hospital laboratories.
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.


METHODS
NNIS System hospital laboratories in the United States.


METHODS
Each laboratory received five organisms (ie, an imipenem-resistant Serratia marcescens, an oxacillin-resistant Staphylococcus aureus, a vancomycin-resistant Enterococcus faecalis, a vancomycin-intermediate Staphylococcus epidermidis, and an extended-spectrum beta-lactamase (ESbetaL)-producing Klebsiella pneumoniae). Testing results were compared with reference testing results from the Centers for Disease Control and Prevention.


RESULTS
Of 138 laboratories testing imipenem against the Serratia marcescens strain, 110 (80%) correctly reported minimum inhibitory concentrations (MICs) or zone sizes in the resistant range. All 193 participating laboratories correctly reported the Staphylococcus aureus strain as oxacillin resistant Of the 193 laboratories, 169 (88%) reported correct MICs or zone sizes for the vancomycin-resistant Enterococcus faecalis. One hundred sixty-two (84%) of 193 laboratories demonstrated the ability to detect a vancomycin-intermediate strain of Staphylococcus 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 for K. pneumoniae, only 98 (51%) of 193 correctly reported this organism as an ESbetaL producer.


CONCLUSIONS
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 ESbetaL-producing organisms is warranted.
Hageman
Jeffrey C
JC
Division of Healthcare Quality Promotion, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA.
Fridkin
Scott K
SK
Mohammed
Jasmine M
JM
Steward
Christine D
CD
Gaynes
Robert P
RP
Tenover
Fred C
FC
D001419:Bacteria / Q000145:classification / Q000187:drug effects / Q000302:isolation & purification; D002983:Clinical Competence; D003428:Cross Infection / Q000175:diagnosis* / Q000188:drug therapy / Q000453:epidemiology / Q000382:microbiology; D004352:Drug Resistance, Microbial; D006801:Humans; D007755:Laboratories, Hospital / Q000592:standards*; D008826:Microbial Sensitivity Tests / Q000592:standards*; D011786:Quality Control; D018571:Sentinel Surveillance; D014481:United States / Q000453:epidemiology