There are currently two available methods for the detection of M. tuberculosis (MTB) infection in the United States. These are: (1) the tuberculin skin test (TST) and (2) the interferon-gamma release assay (IGRA). The TST demonstrates the existence of host hypersensitivity to the tuberculosis bacillus (TB) proteins, usually as a result of MTB infection although it can also be induced with the Calmette–Guérin bacillus (BCG) or infection with environmental mycobacteria. TST provokes an inflammatory reaction with a significant dermal infiltrate at the site of tuberculin inoculation. TST requires the use of 5 PPD units. The result is expressed in terms of millimeters of induration but its interpretation is complex and depends on many variables that may influence not only the size of the reaction but also the development of false negative and false positive results, especially in cases that have been previously immunized with BCG. Until recently, there were no alternatives to TST for LTBI diagnosis. But now, there is the option of a new in vitro test: IGRA. Current evidence suggests that IGRA, especially if based on RD1 antigen cocktails, will potentially be a useful diagnostic test in the clinic and in the public health field. If this potential is attained in practice, it will have to be confirmed in large, well-designed trials and after long-term follow-up.
KeywordsIGRA Immune response Patient immunosuppressed TB infection TST Tuberculin
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