One problem that the clinical laboratory can address for its own purposes of quality control is laboratory contamination. One example is the genotypic analysis of atypical mycobacteria, most notably the M. avium complex (MAC), that has been useful for investigating routes of acquisition from environmental sources and confirming contamination of laboratory cultures (Arbeit et al., 1993; von Reyn et al., 1994; Falkinham, 1996). There are no consensus PCR-based methods for typing Mycobacterium species other than M. tuberculosis. RFLP is widely used for M. avium subsp. avium strains, most of which carry the insertion element IS7245-RFLP (van Soolingen et al., 1998); however, like IS6770-RFLP it requires large cell mass and is labor intensive. Pulsed-field gel electrophoresis (Arbeit et al., 1993; von Reyn et al., 1994) also requires a large cell mass, and few laboratories have the required equipment. Not surprisingly given the limited molecular epi-demiological tools available for environmental mycobacteria, our understanding of the routes of acquisition of these opportunistic pathogens remains limited as does the ability to detect contamination.
Compared with solid media, broth-based mycobacterial culture systems have increased sensitivity but also have higher false-positive rates due to cross-contamination. False-positive cultures can be identified by careful documentation of specimen data and good communication between clinical and laboratory staff. Automated broth culture systems should be supplemented with molecular analysis such as VNTR, so patients are not placed on unnecessary tuberculosis therapy or cases are not falsely identified as treatment failures (Gascoyne-Binzi et al., 2001). When unusual organisms are identified from multiple patients, concern about laboratory contamination, nosocomial spread, or even the possibility of a novel organism associated with disease are raised (Zhang et al., 2002). Advances in molecular typing methods may provide additional tools to help discern the answers to the above questions.
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