Diffuse alveolar hemorrhage (DAH) is a rare but potentially life-threatening manifestation of antiphospholipid syndrome (APS), with poorly characterized pathogenesis, clinical features, and management.

To describe the clinical characteristics, outcomes, and management of APS-associated DAH, and to compare these patients with APS patients without DAH to identify potential risk factors and prognostic markers.

We conducted a retrospective study of APS patients diagnosed with DAH at a French tertiary referral center between 2002 and 2025. DAH was confirmed by clinical, radiological, and/or bronchoscopic findings. Patients were compared with a previously described cohort of 427 APS patients without DAH. Demographic data, clinical features, laboratory findings, imaging, treatments, and outcomes were analyzed.

Sixteen patients experienced at least one DAH episode; 75% were women, with a median age of 30 years [22–42]. Compared with controls, DAH patients had significantly higher rates of hypertension, thrombocytopenia, arterial thrombosis, pulmonary microthrombosis, and catastrophic APS (CAPS). Chest computed tomography consistently showed ground-glass opacities during acute episodes. Most patients required ICU admission (81.3%), and 25% required mechanical ventilation. Treatments included corticosteroids (87.5%), immunosuppressive agents (50%), and continuation of anticoagulation (87.5%). No deaths occurred during DAH episodes. In multivariate analysis, CAPS—but not DAH—was independently associated with mortality; excluding CAPS patients, DAH did not significantly affect survival.

DAH in APS is rare, frequently associated with multi-organ involvement, and presents with severe acute manifestations but favorable short-term outcomes when appropriately managed.