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) using 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 samples from BACTEC vials were tested by the three methods.

RESULTS

Of 24 smear-positive culture-positive (SP-CP) and 11 smear-negative culture-positive (SN-CP) TB specimens, PCR detected 83% and 64%, respectively. Among 17 specimens yielding mycobacteria other than tuberculosis (MOTT), the PCR was positive 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 culture-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, especially in SN-CP specimens, a PCR-based assay utilizing these primers cannot be used reliably, alone, in clinical laboratory diagnosis of mycobacterial respiratory infections.