False-Positive Mycobacterium tuberculosis Culture Revealed by Restriction Fragment Length Polymorphism Analysis

Background: The microbiological analysis of respiratory specimens is the most reliable approach to diagnose active pulmonary tuberculosis. Patient and Methods: We report a 60-year-old female patient (index patient) who underwent diagnostic bronchoscopy for chronic cough. No acid-fast bacilli were detected in bronchial washings. Although cough subsided with symptomatic treatment, Mycobacterium tuberculosis grew on egg-based media after 12 weeks. A false-positive culture result was suspected. Chart review and DNA fingerprinting were carried out. Results: The bronchoscope used to examine the index patient was previously used for a 30-year-old patient (source patient) with smear- and culture-positive pulmonary tuberculosis. Restriction fragment length polymorphism (RFLP) analysis based on the IS6110 element confirmed that the two strains were identical. Conclusion: Cross-contamination is a reason for falsepositive cultures with M. tuberculosis and should be suspected in patients with a low clinical probability for active tuberculosis. Received: March 28, 2002 · Revision accepted: July 5, 2002 O.D. Schoch (corresponding author)