False-positive Mycobacterium tuberculosis culture revealed by restriction fragment length polymorphism analysis

The microbiological analysis of respiratory specimens is the most reliable approach to diagnose active pulmonary tuberculosis.

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.

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 IS 6110 element confirmed that the two strains were identical.

Cross-contamination is a reason for false-positive cultures with M. tuberculosis and should be suspected in patients with a low clinical probability for active tuberculosis.