The United States has seen a dramatic decrease in the incidence of active tuberculous disease (TB) over the past several decades. However, the rate of decrease has recently slowed in large part due to active TB cases in persons born outside the US, who now account for approximately 60% of all cases. Approximately 10% of persons with active TB are known to be co-infected with HIV. The patients at risk for active tuberculosis in a given healthcare facility can vary markedly according to the population served.

M. tuberculosis is usually transmitted only through air, and not by surface contact. It is carried in airborne particles called droplet nuclei that can be generated when persons who have pulmonary or laryngeal TB disease cough, sneeze or shout. Normal air currents can keep them airborne for prolonged periods of time and spread them throughout a room or building. After the droplet nuclei reach the lungs’ alveoli, local infection can be established, with potential for both dissemination to draining lymphatics and hematogenous spread throughout the body. The probability that a person who is exposed to M. tuberculosis will become infected depends on the concentration of infectious droplet nuclei in the air and the duration of exposure to a person with infectious TB disease. Once droplet nuclei fall onto a solid surface they rarely (if ever) become resuspended in the air so objects and surfaces in the vicinity of a patient with active pulmonary disease are not considered capable of transmitting infection, and do not require special handling.

After initial infection, the immune response usually limits additional multiplication of the tubercle bacilli. However, some bacilli remain in the body and are viable for years. This condition is referred to as latent tuberculosis infection (LTBI). Persons with LTBI are asymptomatic and cannot spread TB, but they can reactivate and develop active TB disease. Reactivation occurs in 5-10% of infected persons over their lifetime with the highest risk in the first several years after infection. When reactivation occurs, the individual becomes symptomatic and is capable of transmitting infection to others. Individuals with impaired immunity (from disease or medications) have a higher rate of progressing to active tuberculosis.

Immunologic test results for M. tuberculosis usually become positive within 2-12 weeks after exposure. In the United States, LTBI has been traditionally diagnosed using a Purified Protein Derivative (PPD)-based Tuberculin Skin Test (TST). The tuberculin skin test should NOT be used to rule out active tuberculosis, as there are too many false positives and false negatives for the test to be reliable.

While the receipt of BCG vaccine, given in some countries outside the US, may cause a false positive PPD TST, a positive PPD in a person vaccinated >10 years before likely represent true infection with M. tuberculosis. When there is uncertainty, interferon gamma release assay (IGRA) blood tests can distinguish a positive TST caused by BCG vaccination from one caused by infection with M. tuberculosis. The CDC no longer recommends routine annual TB testing of healthcare personnel; such testing should only be done if there is ongoing transmission at a healthcare facility or if there are particular groups at high risk.