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.


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.
Schoch
O D
OD
Department of Internal Medicine, Division of Pneumology, Kantonsspital St. Gallen, CH-9007 St. Gallen, Switzerland. otto.schoch@kssg.ch
Pfyffer
G E
GE
Buhl
D
D
Paky
A
A
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*