CLUSTER OF ENTEROBACTER-CLOACAE PSEUDOBACTEREMIAS ASSOCIATED WITH USEOF AN AGAR SLANT BLOOD CULTURING SYSTEM

From 1 February through 12 October 1990, 27 blood cultures processed a t Shiprock Hospital were positive for Enterobacter cloacae; only 3 had been reported in the preceding 12 months. Twenty (74%) of the culture s were obtained from patients without clinical evidence of gram-negati ve septicemia. The increase in E. cloacae-positive blood cultures was temporally associated with the introduction of a new blood culturing s ystem. To evaluate potential risk factors for an E. cloacae-positive b lood culture (case-culture), we conducted a case-control study. Case-c ultures were compared with 81 randomly selected cultures that were pro cessed during the epidemic period and that were not positive for E. cl oacae (controls). Because several factors suggested the possibility of pseudoinfection, we limited our analysis to the 20 blood cultures tha t appeared to be contaminants. Blood samples received in the laborator y during the midnight shift (5 of 20 [25%] versus 5 of 81 [6%]; odds r atio, 5.1; 95% confidence intervals, 1.01 to 24.6; P = 0.02) or presen t in the incubator with other E. cloacae-positive samples (17 of 20 [8 5%] versus 29 of 81 [36%]; odds ratio, 10.2, 95% confidence interval, 2.6 to 57.3; P < 0.001) were at increased risk for contamination. Duri ng mock experiments of the procedures for processing blood samples for culture, several breaks in aseptic technique and leakage from the blo od culturing system were observed. Cultures of samples obtained from s everal environmental sites in the laboratory and the hand washings of two laboratory technicians grew E. cloacae. Plasmid and restriction en zyme analyses of E. cloacae isolates recovered from the patients' bloo d cultures, the two technicians' hand washings, and environmental site s in the laboratory indicated that all had identical plasmid profiles. Our findings suggest that the breaks in aseptic technique and the env ironmental contamination that occurred in association with the use of the new blood culturing system resulted in contamination of the blood cultures. This outbreak highlights the importance of routine environme ntal cleaning, periodic quality control assessments, and adherence to aseptic practices in clinical laboratories, particularly when new meth ods or equipment are introduced and/or new personnel are hired.