Tuberculosis

1. Diagnosis of TB

    

   1. Modalities:
      1. Quantiferon: interferon-gamma release assay that detects T cell response to mycobacterial antigens
         1. Used to diagnose latent TB, but does not distinguish between latent and active disease
         2. Marker of immune response, and not microbiologic test for infection
         3. Main advantage over PPD is increased specificity (not affected by BCG vaccination)
         4. Significant false positive rate in active infection, due to anergy of acute illness
      2. AFB smear: this is what we typically order to rule out pulmonary TB
         1. Sensitivity of one sample is ~50%, which is why we order three samples
         2. Negative result ensures that patient is not infective, but is inadequate in ruling out pulmonary TB definitively
      3. AFB culture: gold standard for diagnosis
         1. Liquid media: faster turnaround time (1-3 weeks)
         2. Solid media: more sensitive, but slower (3-8 weeks)
      4. Nucleic acid amplification: rapid turnaround (1-2 days), and should be performed in high risk patients with negative smear
         1. In negative smear, sensitivity 70% (up to 90% with 3 samples)
   2. Remember that the above are for diagnosing pulmonary TB only, and do not apply to systemic disease

Pearl: Endobronchial TB (active infection of tracheal/bronchial airways) can be diagnosed from expectorated sputum or bronchoscopy similar to other forms of pulmonary TB. AFB smear positivity with extensive endobronchial involvement is around 15 to 20 percent per sample of expectorated sputum.

Treatment of TB

1. Initial empiric therapy is 4-drug RIPE cocktail (rifampin, INH, pyrazinamide, ethambutol), to reduce risk of developing resistance

2. Once two months complete, then narrow to two drugs based on sensitivities for duration of therapy, which is typically four months

3. During therapy, should monitor AFB smears at initiation of therapy and after initial phase – if persistently positive, portends increase risk of treatment failure, warranting more prolonged course

4. Remember that INH, rifampin, and pyrazinamide are hepatotoxic

(Ellen Eaton MD, 4/20/11)

(Christopher Woo MD, 1/11/11)