Tuberculosis Testing



LATENT TB

• LTBI is diagnosed when an asymptomatic patient has a positive TST or IGRA result with no clinical or radiographic manifestations of active tuberculosis.

• The goal of diagnosing and treating LTBI is to decrease the risk for reactivation tuberculosis.

• The lifetime risk of developing active infection in patients with LTBI is 5% to 10%.



IGRA/QuantiFERON-TB Gold (QFT)

• 5 y/o + low or intermediate risk of disease progression

• History of BCG vaccination

• Unlikely to return to have their TST result interpreted



Tuberculin skin testing (TST)

False-negative TST result may occur in patients with recent:

• Tuberculosis infection

• Recent viral infections

• Severely immunocompromised (e.g., AIDS)

• < 6 years of age



ACTIVE TB

AFB smear microscopy

• Testing three specimens is highly recommended because false-negative results from a single specimen are not uncommon.



Mycobacterial culture

• Gold standard

• Liquid and solid cultures



NAAT

• Highly recommended to verify the organism if AFB +.

• The positive predictive value of a NAAT on a smear-positive sputum sample = 95%

• A negative NAAT result cannot be used to exclude pulmonary tuberculosis



NAAT + AFB-positive sputum

• Improves diagnostic accuracy



Extrapulmonary infection

• AFB stain

• Mycobacterial culture

• Histopathology demonstrating caseating granulomas



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