Saturday, September 10, 2011

Diagnosis of latent TB

Mantoux skin test
There is no gold standard test for LTBI. For about a century the tuberculin skin test (TST) was the only available test for the detection of LTBI, but the TST has its limitations, including cross-reactivity to bacille Calmette-Guerin (BCG) vaccine and several non-tubercular mycobacteria infections and the need for a second visit, which often results in missed follow-ups (Nahid et al., 2006).
Interferon gamma IFN-γ release assays:
Interferon gamma IFN-γ release assays for diagnosis of LTBI are based on testing the release of IFN-γ by T lymphocytes in response to specific M. TUBERCULOSIS secreted antigens. Two approved commercial IFN-γ tests are essentially available; QuantiFERON-TB gold IN TUBE assay (QFN-G-IT) detecting the released IFN-γ by ELISA, while T-SPOT.TB assay detects the number of IFN-γ producing T cells by means of an enzyme-linked immuno-spot assay (ELISPOT) (Demkow et al., 2008).
The recent use of the M. TUBERCULOSIS early-secreted antigenic target protein (ESAT-6) and the culture filtrate protein (CFP-10) and TB7.7, which are absent in the BCG strain and in the majority of non-tuberculous mycobacteria has improved the specificity of the tests. Because most HCWs worldwide have been vaccinated with BCG, IFN-γ release assay is more accurate than TST in screening of HCWs for LTBI. Also the assays are more objective than TST, requiring only a single laboratory visit with the results can be obtained in the same day (Alvarez-León et al., 2009).

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