Anti-Myeloperoxidase (MPO)-Positive Granulomatosis With Polyangiitis Presenting With Pulmonary and Cutaneous Vasculitic Flares in End-Stage Renal Disease: A Diagnostic and Therapeutic Challenge.

Granulomatosis with polyangiitis (GPA) is a systemic small-vessel vasculitis typically associated with anti-proteinase-3 antibodies. Anti-myeloperoxidase (MPO) positivity is uncommon in GPA and often creates clinical overlap with microscopic polyangiitis. Managing atypical flares in patients with end-stage renal disease (ESRD) is particularly complex, as traditional goals such as renal recovery are absent, shifting the focus to life-threatening extrarenal manifestations. We present the case of a 50-year-old woman with known anti-MPO-positive GPA and ESRD on peritoneal dialysis who presented with progressive dyspnea, hemoptysis, fever, and painful cutaneous blistering on the extremities. Vital signs were notable for fever and mild tachypnea. Laboratory evaluation revealed a creatinine of 11.6 mg/dL, blood urea nitrogen of 55 mg/dL, elevated anti-MPO titers (2.3 IU/mL; normal 0-0.9), and a perinuclear antineutrophil cytoplasmic antibody titer of 1:80. Imaging demonstrated bilateral multifocal ground-glass opacities, tree-in-bud nodules, and scattered pulmonary nodules. Notably, bronchoscopy with bronchoalveolar lavage (BAL) was negative for diffuse alveolar hemorrhage and infection, which could have delayed the diagnosis. However, a biopsy of the cutaneous lesions confirmed leukocytoclastic vasculitis. Based on the integration of serologic, radiologic, and histopathologic findings, a systemic vasculitic flare was diagnosed. The patient was treated with pulse-dose methylprednisolone followed by rituximab induction (375 mg/m² weekly × 4). Cyclophosphamide was intentionally avoided due to her advanced renal disease and side effect profile. She achieved rapid clinical remission, with complete resolution of hemoptysis and healing of skin lesions. This case illustrates that severe MPO-positive GPA can manifest even in dialysis-dependent patients. A key clinical point is that negative BAL findings do not exclude active pulmonary vasculitis, necessitating a multimodal diagnostic approach. Furthermore, rituximab represents a safe and effective induction strategy in high-risk patients where renal salvage is no longer the objective, emphasizing the need for highly individualized management in atypical vasculitic presentations.