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Modern Methods of Tuberculosis Diagnosis

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In low- and middle-income countries, pulmonary active TB usually diagnosed by patient’s appropriate history, clinical symptoms and/or microbiologic evaluation of Mtb by sputum-smear microscopy, culture techniques and chest X-ray. Sputum microscopy involves the examination of Ziehl-Neelsen-stained slide under light microscope. Individuals harboring the largest numbers of acid-fast bacilli in their sputum are considered the most infectious. Although direct microscopy has the advantages of being simple and inexpensive test but lack sensitivity, especially when the number of bacteria is in less amount in the tested sputum as in case of children and in very debilitated individuals. Use of fluorescent microscopy can increase the sensitivity but this in turn become expensive. Microbial culture also remains as an important inexpensive traditional diagnostic test but they also have some limitation of Bio safety lab 3 (BSL3), time consuming and require rapid transportation of sample to the laboratory.

LTBI is indirectly diagnosed by the presence of a specific cellular immune response directed towards mycobacterial antigens in the absence of clinical disease such as tuberculin skin test (TST) or. TST was discovered in 1907 and is an important contributor in the decline of TB in western world and still remain the most cost-effective test. This epidemiological surveillance method, is currently disseminated throughout the world, is based upon the intradermal injection of purified protein derivative (PPD). The PPD solution contains different TB antigens.

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A patient who mounts the cell-mediated response to tuberculin antigen has a delayed-type hypersensitivity response usually within 48-72 hours, causing measurable induration at the injection site, due to migration of mononuclear cells to the area and inflammation process secondary to the cells. The test should be performed by experienced person as the result, depends upon exact intradermal injection and it is difficult to interpret. This response can be attributable to the infection of Mtb, exposure to NTM, or receipt to BCG vaccine. In 2011, Interferon-gamma release assays (IGRAs) was introduced. It is also an immunological based test involving the detection of IFN-γ, a typical cytokine released by lymphocytes in response to three important mycobacterial virulent proteins i.e. ESAT-6, CFP-10 anf TB7.7. The majority of new diagnostic technology in the pipeline are molecular tests that are being developed in reference or intermediate laboratory level only. Since the global plan was introduced by WHO many new molecular diagnostic tests have emerged that have the technical capacity to overcome the limitations of conventional laboratory techniques, discussed earlier. The majority of molecular tests have been aimed at the detection of Mtb specific nucleic acids, both in DNA and RNA, by using amplification techniques such as loop-associated isothermal amplification (LAMP) and simultaneous amplification testing (SAT).

The major advantage of molecular detection modalities is speed of detection, high sensitivity and ease of use. Xpert Mtb/RIF is also a highly specific and sensitive method involves the detection of genes mutation that are related with the resistance to rifampicin by sequencing and nucleic acid hybridization. Urine lipoarabinomannan (LAM) lateral flow assay are used in the diagnosis of TB in HIV-infected patients. Overall, there is still a need of community-based, cost effective, sensitive, user-friendly and rapid diagnostic test.

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