Mycobacterium tuberculosis specimen contamination revisited: the role of laboratory environmental control in a pseudo-outbreak.

OBJECTIVE: To investigate suspected pseudo-outbreaks of Mycobacterium tuberculosis (MTB) during August 1994 and July 1995 among patients who did not have clinical findings consistent with tuberculosis. DESIGN: Retrospective and prospective surveys of all clinical and laboratory data using standard epidemiological tools and DNA fingerprinting. SETTING: A university-affiliated community hospital. PATIENTS: Those with positive MTB cultures during periods when we noted that the number of MTB positive cultures greatly outnumbered the usual monthly average (retrospective analysis, 1994) and patients with positive MTB cultures without clinical findings consistent with tuberculosis (prospective survey, 1995). RESULTS: Epidemiological and molecular studies revealed specimen cross-contamination in the laboratory due to a faulty exhaust hood. Improvement in laboratory ventilation and change of the implicated hood prevented further specimen contamination. CONCLUSIONS: The identification of positive MTB cultures from patients without clinical evidence of tuberculosis should be a signal to suspect laboratory contamination and implement control measures. These should include a thorough epidemiological investigation, DNA fingerprint analysis, and an environmental inspection.