Insight into diagnosis of pleural tuberculosis with special focus on nucleic acid amplification tests

ABSTRACT

Introduction

Pleural tuberculosis (TB) is the archetype of extrapulmonary TB (EPTB), which mainly affects the pleural space and leads to exudative pleural effusion. Diagnosis of pleural TB is a difficult task predominantly due to atypical clinical presentations and sparse bacillary load in clinical specimens.

Area covered

We reviewed the current literature on the globally existing conventional/latest modalities for diagnosing pleural TB. Bacteriological examination (smear/culture), tuberculin skin testing/interferon-γ release assays, biochemical testing, imaging and histopathological/cytological examination are the main modalities. Moreover, nucleic acid amplification tests (NAATs), i.e. loop-mediated isothermal amplification, PCR/multiplex-PCR, nested-PCR, real-time PCR and GeneXpert® MTB/RIF are being utilized. Currently, GeneXpert Ultra, Truenat MTB^TM, detection of circulating Mycobacterium tuberculosis (Mtb) cell-free DNA by NAATs, aptamer-linked immobilized sorbent assay and immuno-PCR (I-PCR) have also been exploited.

Expert opinion

Routine tests are not adequate for effective pleural TB diagnosis. The latest molecular/immunological tests as discussed above, and the other tools, i.e. real-time I-PCR/nanoparticle-based I-PCR and identification of Mtb biomarkers within urinary/serum extracellular vesicles being utilized for pulmonary TB and other EPTB types may also be explored to diagnose pleural TB. Reliable diagnosis and early therapy would reduce the serious complications associated with pleural TB, i.e. TB empyema, pleural fibrosis, etc.

KEYWORDS:

• ALISA
• biomarker
• circulating cfDNA
• diagnosis
• histopathology
• imaging
• Immuno-PCR
• Mycobacterium tuberculosis
• NAATs
• pleural TB

Acknowledgments

The authors are thankful to Krishna B. Gupta, Ekta Kamra and Bhawna Dahiya for critically reading the manuscript and giving valuable suggestions. Bhawna Dahiya also helped us to write inclusion/exclusion criteria in ‘Materials and Methods’ of this study.

Article highlights

• Diagnosis of pleural TB or TB pleural effusion (TPE) is difficult primarily owing to atypical clinical presentations and low bacterial load in the clinical specimens.

• Bacteriological tests (smear/culture) of pleural fluids mostly revealed low sensitivities, although culture of pleural biopsies on MGIT-960 medium exhibited better sensitivities. Markedly, TPE diagnosis relies on ADA detection and other biomarker detection in pleural effusions, imaging, histopathological examination (HPE) and nucleic acid amplification tests (NAATs). Other biomarkers include IFN-γ, LDH, IL-27, CA-125, TNF-α, lysozyme, etc. but most of them exhibit inconclusive results.

• Among several NAATs, multiplex-PCR (M-PCR) targeting mpt64+IS6110 or mpt64+IS6110+pstS1 on pleural fluids/biopsies revealed good diagnostic yield. Further, loop-mediated isothermal amplification (LAMP) targeting IS6110, IS1081 or gyrB showed low to moderate sensitivities but high specificities, while rpoB LAMP showed good sensitivity but moderate specificity.

• GeneXpert revealed a low sensitivity but high specificity within pleural fluids/biopsies, thus Xpert could be used as an excellent rule-in test but not as a good rule-out test, whereas Xpert Ultra revealed somewhat better sensitivity than Xpert. Furthermore, Truenat MTB^TM exhibited higher sensitivities with pleural biopsies, as compared to pleural fluids.

• Detection of circulating Mycobacterium tuberculosis (Mtb) cell-free DNA (cfDNA) by IS6110 real-time PCR within pleural fluids of TPE is another useful approach that demonstrated a significantly higher sensitivity than Xpert, though high specificity was attained with both the assays. Identification of Mtb cfDNA within easily available samples, i.e. plasma/urine of TPE cases needs further optimization that may attain a good diagnostic yield and translate into a point-of-care (POC) test.

• Recently, aptamer-linked immobilized sorbent assay (ALISA) and immuno-PCR (I-PCR) have been used to diagnose TPE with good sensitivities. Moreover, real-time I-PCR (RT-I-PCR)/nanoparticle-based RT-I-PCR being utilized for diagnosing pulmonary TB (PTB) and other EPTB types, may also be explored to diagnose TPE. Identification of Mtb biomarkers within urinary/serum extracellular vesicles (EVs) of TPE cases by I-PCR/RT-I-PCR could be another useful perspective that may exhibit high diagnostic efficacy.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosure

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was funded by the Council of Scientific and Industrial Research (CSIR), New Delhi. PK Mehta acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi for awarding the Emeritus Scientist Fellowship [21(1068)/19/EMR-II] to pursue this study. AS acknowledges the DCRUST-Murthal for awarding the University Research Scholarship.