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- Tuberculosis (TB) is a common infection transmitted by inhaling airborne bacilli from a person with active TB. The bacilli multiply in the alveoli and are carried by macrophages, lymphatics, and blood to distant sites (e.g., lung, pleura, brain, kidney, bone). Tissue hypersensitivity halts infection within 10 weeks.
- Latent TB infection (LTBI) is asymptomatic, noninfectious, and usually detected by a positive skin test (i.e., purified protein derivative [PPD]) or chest x-ray (CXR), which is evidence of prior TB infection; acid-fast bacilli smear and culture are negative, and CXR does not suggest active TB.
- TB: Active disease occurs in 5–10% of infected individuals without preventive therapy. Chance of disease increases with immunosuppression and is highest for all individuals within 2 years of infection; 85% of the cases are pulmonary, which is infectious.
- LTBI treatment is a key component of the TB elimination strategy of the US. For example, an estimated 4,000–11,000 active TB cases were prevented by LTBI treatment in 2002 (1).
A key alert to health care providers is to test for LTBI (PPD) and treat latent infection, with special attention to persons born in countries with high incidence. In the US, foreign-born persons have 12 times the rate of active TB as the rate of US-born persons, with over half of these persons from Mexico (21.5%), Philippines (11.5%), Vietnam (8.2%), India (7.6%), and China (5.6%) (2)
- System(s) affected: Asymptomatic
- High-risk groups include immigrants from Asia, Latin America, Africa, and the Pacific Basin; homeless persons; persons with a history of drug use or history of incarceration; HIV-infected individuals.
- Also at high risk are those newly exposed, including the pediatric population.
- In the US, 4% of the population has latent TB infection.
- Worldwide, it is estimated that 1/3 of the population harbors latent TB.
- Predominant gender: Male > Female
- HIV infection, immunosuppression
- Immigrants (from Asia, Africa, Latin America, Pacific Islands, or area with high rate of TB), including migrant workers
- Close contact with infected individual
- Institutional environment (e.g., prison, nursing home)
- Use of illicit drugs
- Lower socioeconomic status or homeless
- Health care workers
- Chronic disease, such as diabetes mellitus (DM), end-stage renal disease, cancer, or silicosis; organ transplant
- Persons with fibrotic changes on CXR consistent with previous TB infection (3)
- Recent TB skin test (TST) converters (3)
- Personnel from mycobacteriology laboratories
- Patients with organ transplants
Treatment of LTBI is preventive: The goal is to decrease the incidence of active TB in the individuals treated and to decrease the risk to close contacts should active TB occur.
Mycobacterium tuberculosis, M. bovis, and M. africanum
Commonly Associated Conditions
- HIV infection (see “Inital Lab Tests”)
- Immune suppression