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FOUNDATION OVERVIEW

Tuberculosis (TB) is an infection caused by the acid-fast bacillus (AFB) Mycobacterium tuberculosis. Transmission of TB is person to person through inhalation of droplet nuclei in the air. The majority of patients infected with M. tuberculosis develops latent TB infection and do not develop active, symptomatic, disease. Approximately 10% of patients develop active disease if not treated with isoniazid. Risk of active disease is highest in the first 2 years after infection. HIV infection is a significant risk factor for progression to active disease. T lymphocytes, especially CD4+ cells, are important in mounting an immune response to TB. Tumor necrosis factor alpha and interferon gamma are cytokines that are involved in the body’s immune response to TB as well.

TB primarily affects the lungs. The clinical presentation of pulmonary TB includes productive cough, fever, weight loss, night sweats, and hemoptysis. Extrapulmonary TB develops in genitourinary, skeletal, central nervous system, and the pericardial system. Disseminated TB occurs in several parts of the body.

Targeted tuberculin skin testing is used to identify patients with TB. The Mantoux test injects a tuberculin purified protein derivative (PPD) intradermally on the inner portion of the forearm. The induration at the injection site is evaluated in 48 to 72 hours. The size of the induration and the patient’s risk determine whether the test is read as positive (Table 30-1). Patients who have previously received the Bacillus Calmette-Guerin vaccine in another country can have a positive skin test due to the vaccine. This vaccination is not recommended for most of the US population. Interferon gamma release assays measure the level of interferon gamma released in a blood sample in response to TB antigens. This test provides a rapid diagnosis confirmation in 24 hours compared to skin testing. The Centers for Disease Control and Prevention has said it can be used in place of skin testing in individuals older than 5 years. Sputum cultures, chest radiographs, and drug susceptibilities are obtained at first suspicion of active disease.

TABLE 30-1Criteria for Tuberculin Positivity by Risk Group

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