Comparative performance of PCR-based assay versus microscopy and culture for the direct detection of Mycobacterium tuberculosis in clinical respiratory specimens in Lebanon

SETTING: American University of Beirut Medical Center, Lebanon. OBJECTIVE: To assess the performance of a polymerase chain reaction (PCR) u sing primers that flank 542 bp within IS6110 in Mycobacterium tuberculosis (TB) vs. microscopy and BACTEC culture, in the diagnosis of tuberculosis. DESIGN: A total Of 82 clinical respiratory pulmonary specimens and 73 sampl es front BACTEC vials were tested by the three methods. RESULTS: Of 24 smea r-positive culture-positive (SP-CP) and 11 smear-negative culture-positive (SN-CP) TB specimens, PCR detected 83% and 64%, respectively. Among 17 spec imens yielding mycobacteria other than tuberculosis (MOTT), the PCR was pos itive in 33% SP-CP and 14% SN-CP specimens. Among the 73 BACTEC vials, PCR was positive in 36 of 38 (95%) yielding culture-positive TB, and in one of 20 (5%) yielding culture positive MOTT. None of the 30 smear-negative cultu re-negative (SN-CN) clinical specimens and 15 of the CN vials were positive by PCR. The overall sensitivity of PCR was 77% and 95% for TB detection in respiratory specimens and BACTEC vials, respectively, and the specificity was 94% in both. CONCLUSIONS: Because a substantial number of TB cases are missed, especiall y in SN-CP specimens, a PCR-based assay utilizing these primers cannot be u sed reliably, alone, in clinical laboratory diagnosis of mycobacterial resp iratory infections.