Fig. 1.

Clinical states of M. tuberculosis infection. This schematic is adapted from the classification of TB by the American Thoracic Society (ATS) (2). ATS class numbers are also indicated, as applicable. Infected individuals are divided into asymptomatic and symptomatic. (i) The asymptomatic group is further divided into subgroups; color codes indicate the relative risk of progression to active disease in each subgroup (green, low; yellow, high). Past TB (inactive TB; class 4) indicates either a history of a previous episode(s) of active TB or abnormal stable radiographic findings and no bacteriological and/or radiographic evidence of current disease. LTBI (class 1) indicates a positive TST/IGRA and no clinical, bacteriological, or radiographic evidence of active disease. LTBI is further divided into incident/recent (<2 years after infection) or prevalent/remote (>2 years postinfection). The preclinical TB/incipient TB group includes asymptomatic individuals found to have developed active disease when examined at later (short-term) times. (ii) The symptomatic group is also further divided into subgroups; here, color codes indicate bacillary load (orange, low; red, high). Clinical TB indicates symptoms and/or radiographic findings suggestive of active TB but no bacteriological evidence of disease. Culture-confirmed TB (class 3) indicates bacteriological evidence of active TB. These patients are further subdivided into smear-negative and smear-positive groups based on sputum smear microscopy (It is noted that the extent of radiographic lung involvement, such as cavitary and noncavitary disease, is often also used to classify patients.).