The current pulmonary tuberculosis (TB) classification system, established by the World Health Organization, is based on the pathogenesis of the disease. Tuberculosis is a common and sometimes fatal infectious disease caused by various strains of mycobacteria. TB primarily affects the lungs but can also affect other organs. The spread of TB occurs through respiratory droplets expelled by an infected person when they cough or sneeze. According to the Centers for Disease Control and Prevention, most TB infections don't show symptoms, and about one in 10 people who are infected progress to active disease. When left untreated, TB can become deadly, killing more than 50 percent of those infected.
Classification 0
Classification 0 is “No TB exposure,” with no infection and a negative tuberculin skin test called a Mantoux test. This classification means that there has been no known history of exposure to individuals infected with TB and that a negative reaction to a tuberculin skin test can be shown.
Classification 1
Classification 1 means that there has been TB exposure to an infected individual; however, a negative reaction to a tuberculin skin test indicates no evidence of current infection.
Classification 2
Classification 2 states that there is a TB infection; however, no current conclusive active disease state exists. This classification shows a documented positive reaction to a tuberculin skin test but a negative bacteriologic study, with no clinical, bacteriologic or radiographic evidence of tuberculosis.
Classification 3
Classification 3 shows a clinically active TB infection. M. tuberculosis is evident and there is significant clinical, bacteriologic or radiographic evidence of a current disease. Additional strains that can cause TB include the M. bovix, M. africanum, M. Microti and M. canetti.
Classification 4
Classification 4 states that there has been active TB in the past, but is no longer clinically active or that there is an abnormal but stable radiographic finding. Unlike the third classification, this classification can include a positive reaction to the tuberculin skin test, or a negative bacteriologic study and no current clinical or radiographic evidence of active TB.
Classification 5
Classification 5 states that an individual could have active TB and that the diagnosis is pending. Based on clinical, bacteriologic or radiographic evidence, the disease should be ruled in or out within three months.
References
- TB Alliance: The Threat of TB
- Centers for Disease Control and Prevention: Diagnostic Standards and Classification of Tuberculosis
- Centers for Disease Control and Prevention TB Elimination--The Difference Between Latent TB Infection and Active TB Disease
- National Institute of Allergy and Infectious Diseases: Tuberculosis
