Investigation of an outbreak of nosocomial infection due to a multiply drug-resistant strain of Pseudomonas aeruginosa

A nosocomial outbreak of Pseudomonas aeruginosa infections which occurred in the Urology Service of a large city hospital was studied. A case-control methodology was used to analyse patients' characteristics and the main risk factors of all cases with a positive culture during the period between March 1987 and March 1988. The usefulness of factor analysis in the definition of a case was examined. There were 74 infections of which 35 (47·3%), had a nosocomial origin. The outbreak took place in December 1987, with a peak incidence of infections of 10·5%, compared with a 2·2% frequency during the preceding months (P < 0·005). Six of the nine infections occurring in that month, were caused by strains resistant to ticarcillin and gentamicin. The epidemic cases had longer hospital stays than the nonepidemic cases (P < 0·038) and occurred more frequently in a specific area of the hospital (P < 0·001). The odds ratio for resistance to gentamicin was 15 (P < 0·018) and that of resistance to ticarcillin, 127 (P < 0·0001). Our results suggest that inaccurate case definitions may produce misleading conclusions. Factor analysis appears to be a useful analytical tool when defining a case. © 1992.