Casanova Pinto, JoãoCosta, Manuel GFernandes, BeatrizRamalheira, Carlos2026-04-232026-04-232025-102168-8184PURE: 161077965PURE UUID: 0f0b6c75-900a-49c6-9ed9-431869d8d3faPubMed: 41189827PubMedCentral: PMC12580941http://hdl.handle.net/10362/202501Copyright © 2025, Casanova Pinto et al.Antiphospholipid syndrome (APS)-associated diffuse alveolar hemorrhage (DAH) results from antiphospholipid-induced endothelial dysfunction, microvascular thrombosis, and complement activation. Management requires balancing thrombosis prevention and hemorrhage control. High-dose corticosteroids are the first-line therapy. We report a case of a 33-year-old male presenting with progressive dyspnea, hemoptysis, and hypoxemia. Initial evaluation revealed thrombocytopenia, anemia, elevated inflammatory markers, and diffuse bilateral ground-glass opacities on a thoracic computed tomography, consistent with DAH. Autoimmune serology confirmed persistent positivity for lupus anticoagulant, anti-cardiolipin IgM, and anti-β2-glycoprotein-I IgM, fulfilling APS classification criteria. The patient developed respiratory failure, requiring mechanical ventilation and venovenous extracorporeal membrane oxygenation (VV-ECMO). High-dose corticosteroids were initiated, leading to gradual improvement. He was successfully extubated and discharged on long-term anticoagulation with warfarin. At four years of follow-up, he remains free of APS-related complications. This case highlights the importance of recognizing APS as a cause of DAH, particularly in patients with unexplained pulmonary hemorrhage. Early diagnosis and aggressive therapy can improve outcomes.1265532eng2023 acr/eular classification criteriaantiphospholipid antibodyantiphospholipid antibody syndrome (aps)catastrophic antiphospholipid antibody syndromediffuse alveolar hemorrhageprimary antiphospholipid antibody syndromepulmonary capillaritisvenous thromboembolismjournal article10.7759/cureus.93814