Approach to interpretation of sputum AFB smear and NAA test for diagnosis of pulmonary tuberculosis disease in adults

AFB: acid-fast bacilli; NAA: nucleic acid amplification; TB: tuberculosis; NTM: nontuberculous mycobacteria.
* A negative NAA result should be accompanied by a negative test for inhibitors that may limit amplification (if the Enhanced Amplified Mycobacterium Tuberculosis Direct Test [E-MTD] assay was used). The Xpert MTB/RIF assay contains a positive control that signals an invalid result if an inhibitor is detected.
¶ An invalid NAA result indicates failure of the assay. An invalid result is usually reported after the test has been repeated using the same specimen. Receipt of an invalid NAA result should prompt a repeat test using a new specimen.
Δ TB remains possible; the organism burden may be too low to meet the sensitivity threshold for detection via NAA or smear.