Tuberculin skin test: review article

Historically, tuberculosis has been the leading cause of death throughout human history. Tuberculosis infection (TB) causes by Mycobacterium tuberculosis that is very dangerous and can affect any parts of the body, especially lungs. Tuberculosis infection still remains a serious threat to human public health due to its contagious nature, capability to stay latent form in host for indefinite time and then appear as active disease. It is estimated that one third of world’s population, nearly 2 billion persons are infected with Mycobacterium tuberculosis. Transmission occurs among people through inhalation of infected droplets. Lungs and especially alveolar macrophage are primary sites of infection. Mycobacterium tuberculosis bacilli by preventing fusion of phagosome with lysosome can remain alive inside the macrophages. Such situation defined as latent infection. In fact, persons with latent tuberculosis infection (LTBI) are only infected with M. tuberculosis without any sign of infectious. Latent infection in compared with active infection is not contagious, but in about 10-5 percent of people will develop active tuberculosis especially in elderly and people who use immunosuppressive drugs. Pulmonary TB is an active form of tuberculosis infection in which bacteria can spread among people by infected droplets. So identifying and treating people with latent TB infection can significantly reduce the progression of latent form to active infection. The tuberculin skin test (TST) is the most widely used test in worldwide that is applied to determine a person who is infected with M. tuberculosis. TST provide valubale information for diagnosis LTBI however its specificity can be reduced by bacillus Calmette-Guérin (BCG) vaccination and infected with non-tuberculous mycobacteria (NTM). In TST test host hypersensitivity responses to Purified protein derivative (PPD) from mycobacterium are evaluated. TST positive reaction indicates the presence of high risk for acquiring TB infection or progression of latent tuberculosis to active form. Previous studies indicated that there is correlation between TST response and subsequent risk of active TB. Experimental evidence has shown that treatment of latent infection in the basis of positive TST reduces the risk of active TB. Although TST is far from gold standard but it's low cost and simplicity make it a suitable laboratory test especially in developing country.