False-Positive Cultures for Mycobacterium tuberculosis
TB Fact Sheet 1b
The definitive diagnosis of tuberculosis (TB) disease depends on the isolation and identification of the etiologic agent, Mycobacterium tuberculosis, in clinical specimens.
Rapid detection methods for M. tuberculosis
Rapid, sensitive laboratory techniques for the growth and detection of mycobacteria have facilitated diagnosis of TB as well as isolation and appropriate treatment of TB patients. Unfortunately, these new rapid diagnostic methods also appear to have increased the possibility of laboratory cross-contamination.
As more laboratories adopt broth media (e.g., BACTEC®, MGIT®, BacT/Alert®),1 cross-contamination of samples may become more common--particularly in communities with a high incidence of TB, where large numbers of specimens are being evaluated for M. tuberculosis.2,3
False positive cultures for M. tuberculosis
To avoid diagnostic errors based on false-positive cultures, evaluate bacteriology results for possible cross-contamination when all three of the following conditions are met:
- • Sputum samples from the individual patient are being repeatedly monitored
- • All smears are negative for acid-fast bacilli (AFB) and only one culture is positive
- • Clinical suspicion for TB is low
Specimens that are negative for AFB and culture-positive for M. tuberculosis are not unusual; in fact, such results are typical for extrapulmonary TB specimens. However, a single positive culture, preceded or followed by all negative smears and cultures from the same site, should be questioned. For help with evaluations of questionable laboratory results, call the Bureau of TB Control, Surveillance Office, at 311. Clinical judgement determines if treatment is needed for active disease.
Laboratory surveillance
The Bureau of TB Control maintains a surveillance system to identify false-positive M. tuberculosis cultures. Laboratory information is obtained on each questionable culture to determine if it was processed close to another positive culture for M. tuberculosis
All clinical and laboratory information is examined by a staff physician at the Bureau of TB Control, who determines - using DNA fingerprint analysis whenever possible - whether the specimen represents a verified case of TB. The physician of record is notified by letter if the culture is determined to be falsely positive.
Notes and References
- Product names are provided for identification purposes only; their use does not imply endorsement by the New York City Department of Health and Mental Hygiene.
- Braden CR, Templeton GL, Stead WW, et al. Retrospective detection of laboratory cross-contamination of Mycobacterium tuberculosis cultures with use of DNA fingerprint analysis. Clin Infect Dis 1997;24:35-40.
- Nitta AT, Davidson PT, de Koning ML, et al. Misdiagnosis of multidrug-resistant tuberculosis possibly due to laboratory-related errors. JAMA 1996;276(24):1980-83.
