LTBI is diagnosed by a purified protein-derivative skin test or TST after excluding active TB disease. The typical cutoff point is 10 mm induration, but for HIV-positive patients, 5 mm induration is considered a positive reaction.
Newer Food and Drug Administration (FDA)-approved in vitro cytokine-based immunoassays for the detection of TB infection are being introduced (QuantiFERON®). This test does not cross-react with the bacille CalmetteGuerin (BCG) vaccine strain (an attenuated strain of M. bovis) or with other nontuberculous mycobacteria (NTM).
Appropriate clinical specimens for diagnostic testing include sputum, bronchial washings, blood, morning-voided urine, and gastric secretions. The gold standard test to assess the presence of mycobacteria in a clinical specimen is culture on solid media, although this takes three to eight weeks to complete. Correct species identification and drug susceptibility testing then are conducted on the isolated organisms. Acid-fast staining of the clinical specimen is fast but less sensitive than culture and does not distinguish between different mycobacterial species.
NAAT based on PCR has intermediate sensitivity and identifies bacteria as members of the MTB complex but cannot distinguish between dead and living organisms; and drug susceptibility testing is not possible. The sensitivity and specificity of nucleic acid amplification is over 95% for AFB-smear-positive samples, but for smear-negative cases, sensitivity ranges from 40% to 77%. Specificity remains over 95%.
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