Isolation of Mycobacterium tuberculosis (MTB) from the clinical specimens of patients with suspected TB remains the gold standard for diagnosis of TB. However, false-positive MTB cultures can occur as a result of laboratory contamination.
After reviewing the medical records of 400 TB cases identified during January 2008 to January 2009 by the infection control unit of a university-affiliated hospital in Taipei, Taiwan, five patients were considered as clinically suspected false-positive cases and were referred to a mycobacteriology laboratory for confirmation. Spoligotyping and mycobacterial interspersed repetitive unit-variable number tandem repeat analyses were performed for all the suspected isolates and all other isolates cultured on the same day as the five suspected isolates.
Three cases were confirmed as false-positive culture cases based on the laboratory investigation. The culture from one of these cases (index case 1) grew multidrug-resistant TB. Another patient (index case 2) received an extended course of anti-TB treatment after he was considered to have failed treatment because of the false-positive MTB culture result. No anti-TB medication was given for index case 3. All three cases with false-positive cultures had only one positive culture specimen among multiple consecutive specimens submitted for cultures. In addition, specimens of the false-positive cultures were all negative for acid-fast smears.
False-positive MTB cultures should be suspected in the following situations: when growth is observed on only one specimen among multiple specimens submitted; when it is positive in only one culture medium, especially in broth; or when there is only one specimen submitted. False-positive MTB cultures can be further confirmed with modern molecular typing techniques.