Tuberculosis, Latent Infection

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 ...