Bilateral Bloody Pleural Effusions With Discordant Cellular Patterns: A Case of Concurrent Parapneumonic Effusion and Dressler Syndrome.

Bilateral pleural effusions typically suggest systemic aetiologies; however, concurrent distinct localised pathologies are rare. A 73-year-old man presented with fever and cough. Chest imaging revealed pneumonia and bilateral pleural effusions developed during antibiotic treatment. The right effusion was neutrophil-dominant, consistent with parapneumonic effusion, while the left was lymphocyte-dominant, suggesting Dressler syndrome triggered by pneumonia-induced systemic inflammation. Both effusions were bloody, an appearance possibly exacerbated by the concomitant use of a direct oral anticoagulant and aspirin. The left effusion recurred months later but significantly improved following ibuprofen treatment, supporting an immune-mediated aetiology. This case highlights that bilateral effusions can arise from different aetiologies and that pneumonia-induced inflammation may trigger a latent immune response related to prior cardiac surgery.