Diagnostic Approach to Extrapulmonary Tuberculosis by Cartridge-based Nucleic Acid Amplification Test

Tuberculosis (TB) is a highly infectious disease causing millions of cases worldwide. Though pulmonary TB is the most common form of infection, extrapulmonary cases are also very rampant and are responsible for a large number of cases. But the diagnosis of extrapulmonary cases is quite difficult because of varied manifestations and the paucibacillary nature of the infection. Cartridge-based nucleic acid amplification test (CBNAAT) is a simple, rapid test that is very efficient in the early diagnosis of these extrapulmonary cases [extrapulmonary TB (EPTB)].

A study was done to establish the usefulness of CBNAAT in the early diagnosis of EPTB cases.

A comparative study was conducted in a rural tertiary care hospital in West Bengal, India, for 8 months (July 2021-February 2022). Samples were collected from different sites like pleural fluid, lymph nodes, cerebrospinal fluid (CSF), pus, ascitic fluid, and tissue aspirate and subjected to both CBNAAT and smear staining and examination under a fluorescent microscope. Positive samples were cultured, examined, and compared.

From 593 samples collected from different sites in suspected cases of EPTB-52 samples were positive by CBNAAT, and six cases showed rifampicin resistance (RIF resistant). Smear staining of the samples by auramine-rhodamine stains and examined under the fluorescent microscope for acid-fast bacilli identifying 33 samples; the rest were negative. Slides showing acid-fast bacilli were cultured on Lowenstein-Jensen media.

Cartridge-based nucleic acid amplification test (CBNAAT) is a very useful assay for the early diagnosis of extrapulmonary cases as it can accurately identify false negative samples by smear microscopy.