Laboratory Diagnosis of Tuberculosis: Conventional and Newer Methods

Tuberculosis is the main cause for loss of life from a single infectious agent known as Mycobacterium tuberculosis (M. tb). Tuberculin skin test (TST) and interferon-gamma release assays (IGRAs) are two approaches available for testing of latent TB infection but none of these can accurately differentiate between active TB disease and TB infection. Emergence of drug resistant tuberculosis particularly multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis is a challenge to eliminate tuberculosis worldwide. The acid-fast bacillus (AFB) smear microscopy remains the foremost frequently used microbiological test for detection of TB, however it can neither differentiate between live and dead bacilli, nor give any information regarding drug susceptibility testing (DST). Phenotypic culture remains the gold standard but is time consuming and requires good infrastructure. Therefore, significant efforts have been made in last few years for advancement in molecular/genotypic technologies like line probe assay (LPA), cartridge based nucleic acid amplification test (CBNAAT), Truenat MTB test, loop-mediated amplification (LAMP) test, whole genome sequencing etc. Some of these methods have been endorsed by the WHO and have proven to improve case detection and management of TB patients. Presently there is no single test which can be used for detection of tuberculosis in all patients. We have to depend upon combination of different tests and see how to place them in diagnostic algorithm so as to appropriately use all the available diagnostic modalities taking into account factors such as the capacity of different levels and requirement of infrastructure/man power.