Microscopic polyangiitis with histopathologic evolution in serial renal biopsies during treatment of idiopathic pulmonary fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive lung disease associated with inflammation implicated in the development of vasculitis, specifically microscopic polyangiitis (MPA). Herein, we report a case of MPA complicated by IPF. A woman in her 70s with a history of IPF treated with nintedanib presented with a serum creatinine level of 2.22 mg/dL, microscopic hematuria, and a serum myeloperoxidase-antineutrophil cytoplasmic antibodies (MPO-ANCA) titer level of 78.5 IU/mL. The initial renal biopsy revealed diffuse tubulointerstitial nephritis without glomerular crescent formation; therefore, prednisolone was initiated. However, the serum MPO-ANCA titer level increased to 91.1 IU/mL after tapering the prednisolone dose. A second renal biopsy revealed pauci-immune necrotizing glomerulonephritis with crescents, confirming MPA. Treatment was adjusted to include a resumed steroid regimen and combination therapy with rituximab and avacopan, resulting in stable kidney function. In conclusion, this case underscores the importance of monitoring serum ANCA titer levels as a surrogate marker for subclinical vasculitis in patients with IPF. The sequential occurrence of tubulointerstitial nephritis followed by crescentic glomerulonephritis suggests a potential progression pattern in MPA, warranting careful clinical and histopathological evaluations.