MOLECULAR STRAIN TYPING OF MYCOBACTERIUM-TUBERCULOSIS TO CONFIRM CROSS-CONTAMINATION IN THE MYCOBACTERIOLOGY LABORATORY AND MODIFICATION OFPROCEDURES TO MINIMIZE OCCURRENCE OF FALSE-POSITIVE CULTURES

Molecular strain typing by restriction fragment length polymorphism an alysis was used to demonstrate that two clusters of Mycobacterium tube rculosis cultures involving six patients resulted from cross-contamina tion in the mycobacteriology laboratory. Contaminated cultures were pr ocessed by the decontamination procedure and were read on the BACTEC i nstrument following acid-fast bacillus smear-positive specimens from p atients with active tuberculosis. Investigation of these episodes sugg ested opportunities for modification of laboratory procedures to minim ize cross-contamination and confirmed the adverse medical and public h ealth consequences of false-positive cultures. Strain-typing results w ere used in decisions regarding patient care, including the curtailmen t of unnecessary treatment in one patient. Molecular strain typing app ears to be a valuable means of identifying false-positive cultures of M. tuberculosis in selected settings.