A 58-year-old man with idiopathic pulmonary fibrosis is admitted to the ICU with an acute exacerbation of interstitial lung disease. He reports 4 days of worsening dyspnea and dry cough, without fevers, chills, night sweats, or other associated symptoms. His examination is notable for diffuse crackles throughout inspiration. He reports that he is originally from India and moved to the United States 20 years ago, where he has lived since. He is placed on high-flow nasal cannula with FiO2 0.5 and flow of 40 L/min. Methylprednisolone 50 mg daily is administered, along with vancomycin, ceftriaxone, and azithromycin. A chest CT reveals basilar predominant honeycombing with new superimposed multifocal ground glass opacities, along with new scattered right upper and middle lobe centrilobular nodules compared with CT scan 1 year prior. A sputum culture is obtained and sent for bacterial, fungal, and mycobacterial cultures along with acid-fast bacilli (AFB) stain. The sputum is found to have 2+ AFB. A nucleic acid amplification test (NAAT) for tuberculosis (TB) is negative. A repeat sputum sample is collected and again results with 2+ AFB and negative TB NAAT with no presence of NAAT inhibitors detected by the laboratory.
What is the best next step and interpretation of this finding?
A. Obtain past records of Bacille Calmette-Guérin (BCG) vaccination as this may cause a false-negative TB PCRCorrect Answer: C
This patient presents with worsening hypoxemia in the setting of structural lung disease and has new findings of ground glass opacities and centrilobular nodules. Sputum testing reveals AFB with a negative NAAT for tuberculosis on repeated specimens. Latent tuberculosis infection should not result in AFB smear positive sputum (answer D is incorrect). NAAT has high sensitivity and specificity for tuberculosis in AFB smear positive sputum samples. Less than 5% of NAAT testing is falsely negative on AFB smear positive samples because of the presence of nucleic acid amplification inhibitors. If the presence of these inhibitors is excluded, repeat AFB smear positive and NAAT negative samples suggest a nontuberculous mycobacterium (NTM) infection (answer C is correct; answer E is incorrect). NTM infection occurs most commonly in patients with structural lung disease and is a chronic, indolent infection. Prior BCG vaccination may cause a reaction to tuberculin skin testing resulting in a false-positive result; it does not affect the performance of TB PCR (answer A is incorrect). Interferon gamma release assays are testing modalities for latent tuberculosis infection and cannot rule in or out active tuberculosis disease (answer B is incorrect).
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