[The natural history of tuberculosis infection and skin tuberculin reaction]

Abstract

Though most often asymptomatic, tuberculous infection induces a delayed hypersensitivity reaction in the host by activating cellular immunity thus rendering the host refractory, "a priori", to a new infection; at least provided that the infecting dose is not massive or that the immune system of the host is not compromised. Less that ten per cent of immuno-competent individuals infected by Mycobacterium tuberculosis will develop tuberculous disease during their life. The intradermal reaction (IDR to tuberculin) in revealing delayed hypersensitivity to Mycobacterial antigens is in the absence of obvious signs, the only means of diagnosing a tuberculous infection in an individual. It is performed in France by an intradermal injection of 0.1 mls (10 U) of Merieux tuberculin. The response is read at 72 hours. In those who have not had BCG vaccination, an area of induration with a diameter of greater than or equal to 10 mm gives a positive result and is evidence of a tuberculous infection. The test is negative if the diameter is less than 5 mm and indeterminate between 5 and 9 mm. These indeterminate reactions may be the consequence of previous BCG vaccination or of a contact with atypical Mycobacteria in the environment. An IDR of greater than or equal to 10 mm less than ten years after BCG vaccination would not permit any discrimination between a reaction to the vaccine or an authentic tuberculous infection. On the other hand, an IDR of greater than 10 mm ten years after BCG vaccination is evidence of renewed contact with wild tubercle bacilli in 88 per cent of cases. In individuals whose immune defence is altered in particular in patients infected with HIV the threshold of positivity for IDR is lowered to 5 mm.