The incidence of active tuberculosis (TB) in Canada is about 4.8 per 100 000 people, and latent TB affects more than 1.5 million people, in about 5%-10% of whom the disease will reactivate.• Tuberculin skin test (TST) is performed to diagnose latent TB, in the absence of clinical symptoms, and may cause an immune reaction that can, uncommonly, reactivate latent TB.