Polymerase chain reaction for detection of Mycobacterium tuberculosis in sputum

The polymerase chain reaction (PCR) was evaluated in a trial which, with respect to the positive-to-negative ratio, approximated the situation of a diagnostic laboratory in a tuberculosis-endemic area. Three hundred sputum samples were included in the study, of which one-third were known to contain mycobacteria as judged by direct microscopy. The repetitive insertion sequence IS6110/IS986 of Mycobacterium tuberculosis was used as a target. The samples were spiked with DNA from a modified IS6110/IS986 sequence, which gives rise to PCR products easily distinguished from PCR products amplified from chromosomal Mycobacterium tuberculosis DNA. This allowed identification of samples that contained substances inhibitory to the Taq polymerase. The detection limit of the assay was 0.05 pg to 0.5 pg of purified Mycobacterium tuberculosis DNA, corresponding to 10 to 100 organisms. The sensitivity and specificity of the PCR was compared with that of conventional microscopy and culture. It was concluded that this method is fast and sensitive, but that culture currently is crucial for assessing viability and thus infectivity.