Anti-MDA5 Antibody-Positive Dermatomyositis-Associated Interstitial Lung Disease With a False-Positive...

Anti-MDA5 antibody-positive dermatomyositis often causes rapidly progressive interstitial lung disease (RP-ILD) with high mortality. We present a 59-year-old man with three weeks of fever and dyspnea whose chest CT images showed bilateral subpleural ground-glass and reticular opacities. A markedly elevated anti-MDA5 titer and skin biopsy confirmed anti-MDA5 anti-positive dermatomyositis-associated RP-ILD. Tacrolimus plus nintedanib was initiated. Initial bronchoalveolar lavage fluid (BALF) testing on admission was unexpectedly positive for Mycobacterium tuberculosis by targeted RNA capture (TB-TRC), precluding cyclophosphamide and prompting plasma exchange, followed by intravenous immunoglobulin (IVIG). Repeat BALF TB-TRC and all sputum cultures remained negative for M. tuberculosis, confirming a false-positive result. On day 28, tofacitinib replaced tacrolimus due to persistent hyperferritinemia. The patient was discharged without home oxygen therapy on day 54. This case highlights the importance of interpreting rapid assays in context and using multimodal therapy - steroids, calcineurin inhibition, antifibrotics, plasma exchange, IVIG, and Janus kinase (JAK) inhibition - for refractory anti-MDA5 RP-ILD.