Tuesday, September 13, 2011

Advances in cellular immunodiagnosis of Mycobacterium tuberculosis

nullThe TST and interferon-γ release assays (IGRA) evaluate in vivo (TST) and ex vivo (IGRA) the presence of persistent mycobacteria-specific T cell responses.183,184 They are indirect marker for past or present infection. TST and IGRA performed on peripheral blood alone cannot distinguish between individuals with LTBI, active tuberculosis or past tuberculosis.185,186
Tuberculin skin test
The TST was developed by the Austrian paediatrician Clemens v. Pirquet as an ‘allergy-test for the diagnosis of tuberculosis in children’.187 It has been the standard test for the immunodiagnosis of tuberculosis since the beginning of the 20th century. Despite the recent invention of IGRA, the TST is still much more widely used as a screening method for the identification of persons with a positive immune response against M. tuberculosis.188
A standard preparation of purified protein derivate (PPD), an extract of the sterile supernatant of M. tuberculosis-cultered filtrate, is administered intradermaly and results in a delayed type hypersensitivity reaction represented by a local skin induration.189 For the best test result reliability, TST reactions in humans are ascertained by the diameter of induration, measured 48–72 h after antigen injection with the ‘ballpoint technique’.190
A recent meta-analysis stated that the overall sensitivity of the TST for active tuberculosis is 77%;191 however, the sensitivity of the test can be dramatically impaired, for example, in infants and toddlers192 as well as in elderly persons,193 in individuals with congenital or acquired immunodeficiencies (e.g. those with HIV infection194–196), patients being treated with corticosteroids197 or other immunosuppressive drugs,198 patients with chronic renal failure,199,200 malnutrition,201,202 cancer203 or overt forms of tuberculosis.204,205 The specificity of the TST is dependent on the BCG vaccination status206 and the immune status of the individual who is tested.191 Cross-reactivity of antigens may result in a positive TST reaction after exposure to NTM207 or following M. bovis BCG vaccination.206,208 TST induration reactions exceeding 15 mm are likely related to tuberculosis or LTBI,209 irrespectively of the BCG vaccination status.210 While the sensitivity of the TST decreases from a cut-off of 5 to 10 and 15 mm, the specificity of the TST increases with the increase of the cut-off used to define a positive induration.211,212 Depending on the level of exposure to an index case in contact tracing and the immune status of the individual, different cut-offs for a positive test reaction have been recommended ranging from 5- to 15-mm induration.213 Recently, results of a phase I trial of a skin test that uses recombinant early secretory antigenic target (ESAT)-6 instead of tuberculin have demonstrated safety and tolerability of such a test.214 In combination with culture filtrate protein (CFP)-10 antigen to increase diagnostic sensitivity, such a skin test could overcome some of the obstacles currently related to the use of the TST. If clinical trials show superiority to the TST this test could be made widely available for the diagnosis of LTBI in resource-limited settings where the use of IGRA is prohibited by their costs and demands for an established laboratory infrastructure.
Interferon-γ release assays
Introduction of IGRA into clinical practice is regarded by many as the most important development in the diagnosis of M. tuberculosis infection over the last decade. IGRA is a coupling of the discovery of antigens ESAT-6 and CFP-10, which are relatively specific to M. tuberculosis,215 and the development of simplified technologies of measuring interferon-γ. There are two commercialized systems for the latter technology. QuantiFERON-Gold (QFT-G) (Cellestis Ltd, Carnegie, Australia216) measures interferon-γ in IU/mL using an enzyme-linked immunosorbent assay (ELISA) and T-SPOT.TB (Oxford Immunotec Ltd, Abingdon, UK217) counts the cells releasing interferon-γ visualized as spots with the enzyme-linked immunospot (ELISPOT) technique. During the last several years, these systems have been approved in various countries and the findings of their diagnostic performance have been accumulated and characterized. The QFT-G test is now available as an ‘in tube’ version (QFT-G-IT), which also includes, in addition to ESAT-6 and CFP-10, the antigen TB7.7. We present a summary of the performance of these systems in various settings, based on review and meta-analysis.191,218 IGRA were originally intended to diagnose LTBI, but because there is no gold standard of tuberculosis infection, the active disease is usually used as a surrogate for the infection when quantifying sensitivity. Specificity is measured in subjects with low risk of M. tuberculosis infection, for example, healthy young subjects without known contact with tuberculosis patients.

Other biomarkers for tuberculosis disease status and diagnosis
Read : http://www.amazon.com/Mycobacterium-Tuberculosis-Laboratory-Diagnosis-ebook/dp/B0056HRJI8

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