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Chapter 5 : Diagnosis of Latent Tuberculosis Infection
This chapter reviews the tuberculin skin test (TST) and the newer blood tests to detect latent tuberculosis infection (LTBI). TST is still the most widely used and available test for the diagnosis of tuberculous infection. Tuberculous infection with Mycobacterium tuberculosis, a cascade of immune responses ensues triggered by activated macrophages and carried out by T cells. Two types of tuberculin preparations have been in use, old tuberculin (OT) and purified protein derivative (PPD). PPD antigen is administered by multiple-puncture tests and the intradermal Mantoux test. The Mantoux test is performed by intradermally injecting 0.1 ml of PPD tuberculin (5 TU) into the skin of the volar aspect of the forearm. A negative reaction to the tuberculin test does not rule out tuberculous infection. A poor technique of administration can result in a falsely negative reaction. Though skin sensitivity usually persists and is lifelong, waning can occur, often with age, resulting in an apparent negative reaction. In such instances reactivity can be accentuated with repeated testing (the booster effect). Two gamma interferon release assays (IGRA) systems using RD1-encoded antigens are currently commercially available for TB detection. An IGRA or a TST may be used without preference for testing recent contacts of persons with infectious pulmonary TB with considerations for follow-up testing. An IGRA or a TST may also be used without preference for periodic screening of persons who might have occupational exposure to M. tuberculosis with considerations for conversions and reversions.
Key Concept Ranking
- Enzyme-Linked Immunosorbent Assay