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Epidemiologic Notes and Reports False-Positive Blood Cultures Associated with Automated Blood-Culture Analyzers -- Massachusetts

From July to September 1981, two Boston, Massachusetts, hospitals reported blood cultures positive for Staphylococcus aureus resulting from cross-contamination of the cultures on automated radiometric blood-culture analyzers. The reports follow.

Report No. 1: Between July 31 and August 6, 1981, 24 blood cultures from 12 patients at the Faulkner Hospital were positive for S. aureus, phage type 94/96. Since only one of the patients had a clinical illness compatible with S. aureus bacteremic infection, microbiology laboratory equipment and techniques were examined to identify a possible cause for the false-positive blood cultures. As part of the evaluation, tubing connecting the needles to both the flush and sampling circuits were cultured; both yielded S. aureus of the same phage type obtained from the contaminated blood cultures.

To confirm the possibility of cross-contamination on the Bactec 640 blood-culture analyzer, a blood-culture bottle was inoculated with S. aureus and processed routinely; bacteria from this bottle were transferred sequentially to test bottles of sterile culture medium sampled on the machine. Evaluation of the blood-culture analyzer by the manufacturer indicated that it was operating satisfactorily. Review of operating procedures for the machine in the laboratory revealed that several recommended operating and quality-control specifications had not been followed. After these procedures were corrected and the contaminated tubing was changed, the cross-contamination experiment was repeated and again resulted in transmission of bacteria by the machine. Further examination of the machine by the manufacturer failed to identify a specific problem; after the analyzer was replaced, however, no further episodes of cross-contamination occurred.

Report No. 2: From September 16 to September 22, 1981, blood cultures from four patients at the Boston City Hospital became postive for oxacillin-resistant S. aureus. An investigation to identify a common source showed that only one patient was infected with this organism; the other three patients were not colonized and had not been exposed to other patients infected with this strain of S. aureus. The four blood cultures had been obtained the same day and processed sequentially on the Bactec 640 automated blood-culture analyzer; they became positive 2, 4, 7, and 8 days, respectively, after being obtained. During an experimental trial using a blood-culture bottle inoculated with the oxacillin-resistant S. aureus, cross-contamination was documented from the initial blood-culture bottle to three of nine bottles subsequently processed by the analyzer in which bubbles of culture medium were present in the neck of the bottle; cross-contamination did not occur to any of the nine bottles in which bubbles were not present.

Review of blood-culture data for the 7 months preceding this episode identified 20 additional episodes of probable cross-contamination; eight involved S. aureus; 10, S. epidermidis; one, Escherichia coli; and one, satellite-forming Streptococcus. In each instance, cross-contaminated cultures became positive later than the true positive blood cultures (mean 4.1 days compared with 1.6 days; t=4.32, p 0.001). Cross-contamination episodes had occurred with all seven technicians who used the analyzer.

Review of laboratory procedures for the blood-culture analyzer indicated the machine was being properly maintained and that the needles were cleaned, checked, and sterilized according to the manufacturer's instructions. Examination of the analyzer by the manufacturer indicated malfunction of the needle sterilizer; after the sterilizing unit and computer board controlling its function were replaced, experimental cross-contamination no longer occurred on the unit, and no further episodes of cross-contamination occurred. Reported by SA Weinstein, MPH, B Weinstein, MD, FO Escobar, BC Fine, MD, Faulkner Hospital, DE Craven, MD, DL Lichtenberg, K Browne, D Coffee, T Treadwell, MD, WR McCabe, MD, Maxwell Finland Laboratory for Infectious Diseases, Boston City Hospital, Boston, NJ Fiumara, MD, State Epidemiologist, Massachusetts State Dept of Public Health; Hospital Infections Program, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: False-positive blood cultures may create unnecessary concern for patients and physicians and may result in additional laboratory tests and costs and in unnecessary treatment of patients. Blood cultures in both hospitals were processed on analyzers that operate by detecting radioisotopically labeled gases produced by microorganisms growing in the special culture medium. The gases are sampled in each blood-culture bottle by two needles that simultaneously puncture the rubber diaphragm of the culture bottle; the gas in the space above the culture medium is flushed into the sampling needle by a mixture of inert gases entering the bottle through the other needle. After sampling each unit, the needles are withdrawn, and the tips are heat-sterilized automatically by the machine before being inserted into the next culture bottle for sampling. The problem resolved after replacement of the needle-sterilizing unit in the analyzer in one hospital and after replacement of the entire analyzer in the other.

Radiometric blood-culture analyzers of the type used in these hospitals are used widely in hospital laboratories; they have been associated with clusters of false-positive blood cultures in the past (1,2). In one instance, cross-contamination apparently occurred because the needle probes on the machine were not changed, cleaned, and sterilized daily (1); in the other, the power unit and circuit boards controlling heat sterilization of the needle probes malfunctioned and had to be replaced (2). Cross-contamination with a variety of organisms apparently can occur, although this is the first documented report of cross-contamination with S. aureus on these machines. The problems were detected through a combination of surveillance and clinical judgment about the incompatability between culture results and patient illness. Report number 1 is also unique in that no specific malfunction of the machine could be identified, and contamination of the tubing attached to the needles occurred. The analyzer cannot sterilize tubing once contaminated. These reports also suggest that bubbles of media in the necks of the culture bottles may greatly increase the probability of cross-contamination.

In some laboratories, use of the radiometric blood-culture analyzers may decrease the time between obtaining a blood culture and detection of bacterial growth. Laboratory personnel must be aware that these machines can malfunction, allowing cross-contamination to occur. Prevention of these problems depends on carefully following recommended procedures for maintenance and operation of the machine and periodic review of laboratory log books to try to detect the occurrence of unusual clusters of positive blood cultures that might be caused by cross-contamination on the analyzer.


  1. Greenhood GP, Highsmith AK, Allen JR, Causey WA, West CM, Dixon RE. Klebsiella pneumoniae pseudobacteremia due to cross-contamination of a radiometric blood culture analyzer. Infect Control 1981;2:460-5.

  2. Griffin MR, Miller AD, Davis AC. Blood culture cross contamination associated with a radiometric analyzer. J Clin Microbiol 1982;15:567-70.

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