An estimated one third of the world’s population is infected with Mycobacterium tuberculosis,1 presenting a major impediment to tuberculosis control. Despite the importance of latent tuberculosis infection (LTBI), the tuberculin skin test (TST) was, until recently, the only test available for its diagnosis.2-5 The TST measures hypersensitivity response to purified protein derivative (PPD), a crude mixture of antigens, many of which are shared among M tuberculosis, Mycobacterium bovis bacille Calmette-Guérin (BCG), and several nontuberculous mycobacteria (NTM). The TST has limitations with respect to accuracy and reliability.2-6