The spectrum of TB — from Mycobacterium tuberculosis infection to active (pulmonary) TB disease: Although tuberculosis (TB) disease can be viewed as a dynamic continuum from Mycobacterium tuberculosis infection to active infectious disease, patients are categorized as having either latent TB infection (LTBI) or active TB disease for simplicity in clinical and public health settings. Individuals can advance or reverse positions, depending on changes in host immunity and comorbidities. Exposure to M. tuberculosis can result in the elimination of the pathogen, either because of innate immune responses or because of acquired T cell immunity. Individuals who have eliminated the infection via innate immune responses or acquired immune response without T cell priming or memory (denoted by *) can have negative tuberculin skin test (TST) or interferon-γ release assay (IGRA) results. Some individuals will eliminate the pathogen, but retain a strong memory T cell response and will be positive on the TST or the IGRA. These individuals will not benefit from LTBI treatment. If the pathogen is not eliminated, bacteria persist in a quiescent or latent state that can be detected as positive TST or IGRA results; these tests elicit T cell responses against M. tuberculosis antigens. These patients would benefit from receiving one of the recommended LTBI preventive therapy regimens (mostly 6–9 months of isoniazid). Patients with subclinical TB might not report symptoms, but will be culture-positive (but generally smear-negative because of the low bacillary load). Patients with active TB disease experience symptoms such as cough, fever and weight loss, and the diagnosis can usually be confirmed with sputum smear, culture and molecular tests. Patients with active TB disease might sometimes be negative on the TST or the IGRA because of anergy that is induced by the disease itself or immune suppression caused by comorbid conditions, such as HIV infection or malnutrition. Individuals with subclinical or active TB disease should receive one of the recommended treatment regimens for active TB disease, which consist of an intensive phase with four drugs, followed by a longer continuation phase with two drugs. | Pai, M., Behr, M. A., Dowdy, D., Dheda, K., Divangahi, M., Boehme, C. C., … Raviglione, M. (2016). Tuberculosis. Nature Reviews Disease Primers, 2(1), 16076. https://doi.org/10.1038/nrdp.2016.76

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