Tuberculin skin testing

TST is a method to detect TB infection that involves intradermal injection of tuberculin purified protein derivative (PPD). Previous exposure results in a local delayed-type hypersensitivity reaction within 24–72 hours (6). The reaction is identified as palpable induration at the site of injection. It only indicates hypersensitivity to proteins of the TB bacillus as a result of infection with M. tuberculosis or induced by BCG vaccination. A positive TST does not indicate the presence or extent of TB disease. A TST reaction after previous BCG vaccination is usually weaker than a reaction to natural infection and will remain positive for several years thereafter. Various clinical conditions, including HIV, may suppress a TST reaction. A negative result does not rule out TB infection or TB disease. In children living with HIV, or those with severe malnutrition or another severe illness, an induration of 5 mm or more is considered positive. For children without these conditions (irrespective of previous BCG vaccination), an induration of 10 mm or more indicates a positive result (6). Annex 2 provides detailed information on administering, reading and interpreting TST.

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