Laboratory cross-contamination of Mycobacterium tuberculosis: an investigation and analysis of causes and consequences.

BACKGROUND: The misdiagnosis of Mycobacterium tuberculosis infection has many ramifications. These include medical and psychological implications for patients and their families and financial and public health implications for health-care institutions. Microbiology laboratory procedures should minimize the possibility of laboratory cross-contamination of specimens and maximize the ability to recognize a cluster of false-positive cultures. Newer molecular typing methods provide rapid, accurate and effective means of identifying false-positive M. tuberculosis cultures. AIMS: To investigate a cluster of patients with positive M. tuberculosis cultures that were processed in the mycobacteriology laboratory on the same day.
METHODS: Five patients' medical records and radiology results were reviewed to determine whether the cases were epidemiologically linked and whether there was clinical suspicion of tuberculosis. Restriction fragment length polymorphism (DNA fingerprinting) was performed using repetitive elements IS6110 and pTBN12. Laboratory processing procedures were analysed.
RESULTS: On the basis of DNA fingerprinting using IS6110, the isolates from all five patients were identical. Molecular typing using pTBN12 was performed on four of the five isolates. All four had identical patterns. There was no epidemiological link between the patients. At least three (and probably four) of the five patients were misdiagnosed with tuberculosis.
CONCLUSION: Microbiology laboratories should ensure that appropriate methodologies are in place to avoid cross-contamination of specimens. Clinicians need to critically interpret any positive laboratory result, especially in an unlikely clinical setting.