Real-Time Polymerase Chain Reaction Assay: A Response to Recent Letter to the Editor

Thank you for the recent comments1 for the topic of real-time polymerase chain reaction (RT-PCR) assay2. We propose that positivity in RT-PCR using any respiratory specimens suggests the possibility of active tuberculosis (TB) in clinically suspected cases, guiding to start anti-TB medication, and RT-PCR from selective bronchoscopic aspirates enhances the diagnostic yield much more when added to sputum examination2. Wiwanitkit1 mentioned that "There are some concerns on this assay. False-positive of the test can be seen in cases with treated or old lesion from pulmonary TB and the low sensitivity of the test can be seen."

As a response to the issues of false-positivity raised by Wiwanitkit1, it was mentioned in our paper2 that "In our study, the false-positive rate was 0.5% in sputum and 2.0% in bronchoscopic aspirates. False-positivity in PCR has been reported to be due to carry-over contamination between specimens, cross-reactions with isolated nontuberculous mycobacteria, or dead tissue debris from previous TB scarring in highly endemic areas."

Low sensitivity of this test was also discussed in our paper2 that "Most reports have evaluated PCR using known acid-fast bacilli-positive samples. In smear-positive specimens, the sensitivity and specificity of polymerase chain reaction are in the range 90%-100%, with a positive predictive value of >95%, whereas in smear-negative specimens, the sensitivity of PCR is reduced to <50%. In this study, the sensitivity of RT-PCR in acid-fast bacilli smear-positive specimen was observed 89%. Factors that affect RT-PCR sensitivity include the individual effort expended for sputum collection and clinician bias with regard to diagnostic approaches."