Light chain proximal tubulopathy in Sjögren syndrome: the critical role of tubular function tests in revealing a hidden threat.

A 65-year-old female patient with Sjögren syndrome (SJS) and tubulointerstitial nephritis (TIN) confirmed by kidney biopsy results 7 years previously presented to our department with progressive kidney function deterioration. Laboratory findings revealed increased serum creatinine level accompanied with deterioration of tubular function. Although she already had signs of proximal tubular dysfunction due to TIN from 7 years before, deterioration of the proximal tubule related parameters was particularly prominent. Owing to the substantial increase in the IgG level, monoclonal gammopathy was considered. Subsequent immunoelectrophoresis of the serum and urine revealed the presence of monoclonal IgG-κ. A repeat kidney biopsy revealed crystalline inclusions in the proximal tubules that tested positive for κ light chain. Along with findings from the bone marrow biopsy, this led to the diagnosis of light chain proximal tubulopathy (LCPT) secondary to multiple myeloma. Chemotherapy was initiated promptly which resulted in stabilization of renal function and improvement of tubular function. Vigilant monitoring of the tubular function is essential for patients with SJS. Deterioration of proximal tubule parameters, especially when accompanied by increased IgG levels, should prompt the investigation of monoclonal gammopathy and the consideration of kidney biopsy.