A69-29 Ground-Glass Mystery: Pulmonary Nocardiosis in an Immunocompromised Host Misattributed to “Steroid-Responsive” Pneumonitis

Abstract Pulmonary nocardiosis is an uncommon, often under-recognized infection that can mimic inflammatory lung disease, especially in patients receiving B-cell-depleting therapy and prolonged corticosteroids.A 56-year-old man with marginal cell lymphoma on maintenance rituximab and a history of prolonged high-dose prednisone for recurrent fever of unknown origin presented with months of cough. He was a former smoker (38 pack-years). Extensive infectious and autoimmune testing was negative (QuantiFERON, coccidioidomycosis, histoplasma, ANCA, RF/CCP, myositis panel). Chest CT demonstrated multifocal ground-glass opacities with scattered nodularity. Bronchoscopy with BAL was performed; modified acid-fast staining revealed branching filamentous organisms suspicious for Nocardia species (cultures pending at the time of treatment initiation). Given intolerance to trimethoprim-sulfamethoxazole, multidrug therapy was started with ceftriaxone, linezolid, and minocycline; minocycline was substituted for TMP-SMX due to diarrhea. Corticosteroids were tapered to physiologic dosing. The patient reported symptomatic improvement, and interval CT imaging was planned to document radiographic response.This case highlights three diagnostic pitfalls. First, diffuse or patchy ground-glass opacities in an immunocompromised patient are frequently attributed to drug toxicity or organizing pneumonitis; however, Nocardia can present with the same pattern without frank cavitation. Second, “steroid responsiveness” can transiently blunt inflammatory manifestations of nocardiosis, delaying diagnosis. Third, TMP-SMX intolerance is common; early use of alternative active agents (e.g., linezolid, ceftriaxone, minocycline) guided by susceptibilities is critical, with treatment often required for ≥6-12 months and concurrent tapering of immunosuppression when feasible.In patients on rituximab and chronic steroids who present with persistent cough and ground-glass opacities, clinicians should maintain a high index of suspicion for pulmonary nocardiosis and pursue early bronchoscopy with appropriate staining/culture to avoid delays, inappropriate steroid escalation, and adverse outcomes. This abstract is funded by: None