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 d ata using standard epidemiological tools and DNA fingerprinting. SETTI NG: A university-affiliated community hospital. PATIENTS: Those with p ositive MTB cultures during periods when we noted that the number of M TB positive cultures greatly outnumbered the usual monthly average (re trospective analysis, 1994) and patients with positive MTB cultures wi thout clinical findings consistent with tuberculosis (prospective surv ey, 1995).RESULTS: Epidemiological and molecular studies revealed spec imen cross-contamination in the laboratory due to a faulty exhaust hoo d. Improvement in laboratory ventilation and change of the implicated hood prevented further specimen contamination. CONCLUSIONS: The identi fication of positive MTB cultures from patients without clinical evide nce of tuberculosis should be a signal to suspect laboratory contamina tion and implement control measures. These should include a thorough e pidemiological investigation, DNA fingerprint analysis, and an environ mental inspection.