Abstract Pneumocystis jirovecii pneumonia (PJP) and pulmonary alveolar proteinosis (PAP) may present with overlapping clinical, radiographic, and histopathologic features, including diffuse ground-glass opacities and periodic acid-Schiff (PAS)-positive intra-alveolar material, creating diagnostic challenges in immunocompromised patients. Common variable immunodeficiency (CVID) primarily manifests as hypogammaglobulinemia with recurrent sinopulmonary infections, but corticosteroid exposure can further impair cellular immunity and increase susceptibility to opportunistic fungal infections. We report a case of severe PJP in a 26-year-old woman with undiagnosed CVID initially misdiagnosed as PAP. She presented with a 3-day history of dyspnea and cough, with imaging showing bilateral patchy infiltrates treated as community-acquired pneumonia. Symptoms progressed over 3 months, leading to hospitalization for hypoxemic respiratory failure, where CT imaging demonstrated extensive bilateral ground-glass and reticular opacities. Infectious evaluation was unrevealing, and a course of corticosteroids was initiated for presumed organizing pneumonia. She was discharged on supplemental oxygen but re-presented three weeks later with worsening hypoxemia and restrictive physiology on pulmonary function testing; bronchoalveolar lavage (BAL) was negative. Surgical lung biopsy revealed PAS-positive material interpreted as PAP. Two weeks later, she was hospitalized again with acute respiratory failure and pneumothorax; repeat BAL demonstrated PAS-positive material and Grocott methenamine silver-positive Pneumocystis cysts. Expert pathology review confirmed PJP without histologic PAP. Immunologic evaluation revealed progressive hypogammaglobulinemia (IgG declining to 148 mg/dL) with elevated CD4, CD8 and CD19 counts on quantitative lymphocyte profile, along with absent antigen-specific and mitogen lymphocyte responses, consistent with combined humoral and cellular immune dysfunction; anti-GM-CSF antibodies were negative. She was diagnosed with CVID complicated by corticosteroid-associated susceptibility to PJP and treated with trimethoprim-sulfamethoxazole, a prednisone taper, and intravenous immunoglobulin, resulting in clinical improvement, radiologic resolution, and successful oxygen weaning. This case highlights that PAS-positive findings alone may misdirect clinicians toward PAP, and emphasizes the value of confirmatory fungal staining, multidisciplinary collaboration, and expert pathology review when imaging or clinical course is atypical. This abstract is funded by: none
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