Tubulointerstitial nephritis in antineutrophil cytoplasmic antibody-associated vasculitis with monoclonal gammopathy.

Isolated tubulointerstitial nephritis (TIN) without glomerular crescent formation is a rare manifestation of antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV). Some patients with monoclonal gammopathy of undetermined significance present with renal complications due to serum monoclonal protein. Here, we present a case of TIN presumably attributable to AAV with monoclonal gammopathy. Laboratory data revealed acute kidney injury, elevated C-reactive protein (CRP) and ANCA titers, and elevated tubular injury markers. Renal biopsy revealed TIN with no apparent glomerular lesion. The findings of peritubular capillaritis and tubulitis indicated that AAV had contributed to the development of TIN. However, in situ hybridization for free light chains revealed kappa light chain restriction, indicating that the involvement of monoclonal gammopathy in the pathogenesis of TIN remains possible. The patient also developed ophthalmic neuropathy, probably caused by AAV. Oral prednisone (0.6 mg/kg/day) administration improved both the ocular symptoms and the laboratory parameters. Our case demonstrated that the concurrence of AAV and monoclonal gammopathy could pose a diagnostic dilemma in distinguishing the cause of TIN. Besides, some reports suggest an association between AAV and monoclonal gammopathy, although direct evidence is lacking. Further research is needed to establish this association.