title: False-positive Mycobacterium tuberculosis culture revealed by restriction fragment length polymorphism analysis.
abstract: BACKGROUND
  The microbiological analysis of respiratory specimens is the most reliable approach to diagnose active pulmonary tuberculosis.
  
  
  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 IS 6110 element confirmed that the two strains were identical.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Internal Medicine, Division of Pneumology, Kantonsspital St. Gallen, CH-9007 St. Gallen, Switzerland. otto.schoch@kssg.ch
authors.forename: O D
authors.initials: OD
authors.lastname: Schoch
authors.forename: G E
authors.initials: GE
authors.lastname: Pfyffer
authors.forename: D
authors.initials: D
authors.lastname: Buhl
authors.forename: A
authors.initials: A
authors.lastname: Paky
mesh_terms: D016172:DNA Fingerprinting; D003937:Diagnosis, Differential; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification*; D012150:Polymorphism, Restriction Fragment Length; D012141:Respiratory Tract Infections / Q000175:diagnosis / Q000382:microbiology*; D013183:Sputum / Q000382:microbiology; D014374:Tuberculin Test; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis*