 The purified protein derivative (PPD) skin test (Mantoux test) is an important tool for diagnosis of latent TB infection and disease in the developed world, but it has low predictive value in Bacillus Calmette-Guérin (BCG)–vaccinated individuals, as well as in individuals living in areas where TB is endemic.The low predictive value is a result of cross-reactivity with BCG and atypical mycobacteria, as well as false negative reactions in malnourished children.BCG has been used as an antigen in EIAs in in vitro studies to determine disease activity, but its use was suspended because of difficulties in interpretation, problems differentiating between active or past disease, and low sensitivity and specificity.