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Key Features

Essentials of Diagnosis

  • Positive tuberculin skin test

  • No evidence of active infection with tuberculosis (TB)

  • History (knowingly or not) of exposure to Mycobacterium tuberculosis (MTB)

General Considerations

  • Targeted skin testing used to identify

    • Persons at high risk for TB

    • Persons who would benefit from treatment of latent TB infection (LTBI)

  • LTBI describes patients who have been infected with M tuberculosis but do not have active disease

  • Individuals with LTBI have contained but not eradicated infection

  • The importance of identifying and treating LTBI is to prevent reactivation disease

  • LTBI is nontransmissible but may become active disease if a person's immune function becomes impaired

  • Active TB will develop in approximately 6% of individuals with LTBI who are not given preventive therapy; half of these cases occur in the 2 years following primary infection

  • Patients with primary infection require 2–10 weeks to manifest an immune response to skin testing

  • Persons who have received bacillus Calmette-Guérin (BCG) vaccination may have a positive purified protein derivative (PPD) tuberculin test for the rest of their lives

  • Resistance may be to single or multiple drugs

    • Drug-resistant TB is resistant to one first-line antituberculous drug, either isoniazid (INH) or rifampin (RIF)

    • Multidrug-resistant tuberculosis (MDR-TB) is resistant to INH and RIF, and possibly additional agents

    • Extensively drug-resistant tuberculosis (XDR-TB) is resistant to INH, RIF, fluoroquinolones and either aminoglycosides or capreomycin or both

Clinical Findings

Symptoms and Signs

  • Patients are asymptomatic

  • LTBI is uncovered by screening with the tuberculin skin test

  • Any pulmonary or constitutional symptoms should prompt an evaluation for active disease prior to prophylactic treatment

Differential Diagnosis

  • BCG vaccination

Diagnosis

Laboratory Tests

  • All patients with risk factors should be tested for HIV

Imaging Studies

  • Chest radiograph is required to rule out active pulmonary TB

Diagnostic Procedures

  • The Mantoux test is the preferred skin test (Table 9–13)

    • 0.1 mL of PPD containing 5 tuberculin units is injected intradermally on the volar forearm

    • At 48–72 hours, transverse width in millimeters of induration is measured

  • False-positive tuberculin skin test reactions occur in patients previously vaccinated against MTB with BCG

  • Prior vaccination with BCG does not alter the interpretation of the tuberculin skin test

  • False-negative tests may result from

    • Improper technique

    • Concurrent infections

    • Malnutrition

    • Advanced age

    • Immunosuppression of any kind

    • Fulminant MTB infection

  • Because of waning immunity, some patients with LTBI may have a negative skin test many years after exposure

  • Two-step testing or "boosting"

    • Performed to reduce the likelihood that a boosted reaction will later be misinterpreted as a recent infection in individuals who will be tested repeatedly (health care workers)

    • A second test is performed 1–3 weeks after a negative test

    • If negative, the patient ...

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