Rituximab and corticosteroids for M-protein-negative PGNMID: case report

We report a case of a 69-year-old patient negative for serum M-protein who was diagnosed with proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) via renal biopsy. A 69-year-old male presented with hematuria, edema, and rapidly progressive renal failure. Serum M-protein was negative, but renal biopsy confirmed IgG3-κ PGNMID with intense C3 deposition and an MPGN-like pattern. Rituximab was initiated, but due to a pulmonary infection, only one of the planned rituximab doses was administered. After infection resolution, the patient’s renal function gradually improved (serum creatinine from 914 to 134 µmol/L) under long-term corticosteroid maintenance. Whether this improvement resulted from the single rituximab dose, prolonged corticosteroids, resolution of sepsis-associated AKI, or natural disease course remains uncertain. This case underscores that PGNMID, though rare, should be considered in seronegative patients with unexplained glomerulonephritis, and highlights the critical role of renal biopsy. PGNMID should be considered in rapidly progressive glomerulonephritis, even when M-protein is negative. In this patient, a single reduced dose of rituximab combined with prolonged corticosteroids was associated with a favorable renal outcome, suggesting that a B-cell–directed, clone-directed approach may be a reasonable option worth further evaluation in seronegative PGNMID. Nonetheless, causality cannot be established from a single case, and the relative contributions of the individual therapies remain uncertain. Further studies are needed to define the optimal treatment for this rare entity.