A PSEUDOEPIDEMIC DUE TO LABORATORY CONTAMINATION DECIPHERED BY MOLECULAR ANALYSIS

OBJECTIVE: A clinical, microbiological, and molecular analysis to iden tify the source of a cluster of pseudoinfections. DESIGN: Retrospectiv e analysis of the cases, prospective epidemiologic survey, and laborat ory investigation. Molecular analysis of the isolates was performed us ing pulsed-field gel electrophoresis (PFGE). SETTING: A tertiary Veter ans Affairs medical center. PATIENTS: Three patients admitted over a 2 -week period with musculoskeletal complaints had one or more joint flu id specimens submitted for culture. In each case, anaerobic chopped me at-glucose broth (CMGB) tubes yielded one or more organisms not typica lly associated with septic arthritis (Enterobacter cloacae, Enterococc us faecium, Enterococcus casseliflavus, Enterococcus faecalis, Escheri chia hermannii, and Pseudomonas diminuti). The first three organisms w ere isolated from specimens from multiple patients. Two patients had m ultiple positive cultures; for two patients, separate cultures yielded additional organisms on solid media. RESULTS: Laboratory investigatio n yielded an isolate of E faecium from 1 of 30 sham-inoculated CMGB tu bes. PFGE analysis demonstrated that a single strain of E cloacae was isolated from four CMGB tubes representing all three patients, and a s ingle strain of E faecium was isolated from CMGB tubes representing tw o patients and the shaminoculated tube. CONCLUSIONS: The application o f molecular typing clearly demonstrated clonality among the isolates a nd indicated that a common source of contamination, most likely the CM GB tubes, was responsible for these cases.