[Selection of relevant duplicate criteria in the monitoring of Staphylococcus aureus methicillin resistance trends].
OBJECTIVE
Monitoring methicillin-resistant Staphylococcus aureus (MRSA) rates is crucial in hospital settings, but its calculation is hampered by variations according to duplicate isolates. The aim of this study was to assess the impact of applying different methods for duplicate isolate removal on the MRSA rates.


METHODS
Trends in S. aureus resistance were analysed from isolates collected in a teaching hospital during a 4-year period by using a specifically designed software. Reference duplicate (RD) criteria were defined as one major and three minor differences in antibiotic patterns and a 30-day time period between two isolates. Variations in antibiotic patterns and time period were studied and compared to RD. NCCLS recommendations based upon results from the first isolate of a species per patient per study period, and the ONERBA recommendations based on phenotypic method, were also studied.


RESULTS
MRSA rate was 31.1% when using RD. When duplicates were not eliminated, the MRSA rate significantly increased to 34.0% (P=0.002). When using NCCLS and ONERBA recommendations, the MRSA rate significantly decreased to 27.6% (P=0.002) and significantly increased to 33.8% (P=0.002) respectively. When no difference was tolerated in antibiotic pattern or when the time period was set at five days, the MRSA rate significantly increased to 34.3 and 34.2%, respectively (p=0.003 for each).


CONCLUSIONS
These results suggest that criteria used to remove duplicates should be carefully selected to assess accurate MRSA trends.
Grohs
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P
Service de microbiologie, hôpital européen Georges-Pompidou, 20-40 rue Leblanc, 75908 Paris cedex 15, France. Patrick.GROHS@hop.egp.ap-hop-paris.fr
Kac
G
G
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D018432:Drug Resistance, Multiple; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D015203:Reproducibility of Results; D012107:Research Design / Q000592:standards; D013211:Staphylococcus aureus / Q000187:drug effects*
D000900:Anti-Bacterial Agents