A 68-year-old male patient visited the hospital for the management of bilateral lower extremities edema. He had a 20-year history of diabetes mellitus, complicated by diabetic retinopathy. Laboratory investigations revealed a serum creatinine level of 0.98 mg/dL, hemoglobin 6.6 g/dL, total protein 6.4 g/dL, albumin 2.9 g/dL, and globulin 3.5 g/dL. Urinalysis indicated significant proteinuria with 3+ protein with hematuria and a urine protein-creatinine ratio of 2,221.59 mg/g. After admission, his renal function slowly declines to serum creatinine of 1.33 mg/dL and worsens bilateral lower extremities edema. To address other possible causes of anemia and proteinuria, we evaluated the serum and urine protein electrophoresis with immunofixation and identified a monoclonal lambda subtype of immunoglobulin M (IgM). Further evaluation included a bone marrow biopsy, establishing a hematological lymphoplasmacytic lymphoma (LPL) diagnosis (Fig. 1). The renal biopsy revealed light chain cast nephropathy alongside diabetic nephropathy. Congo red staining yielded negative results, ruling out amyloidosis (Fig. 2). Consequently, a diagnosis of cast nephropathy in the context of LPL was confirmed. We treated with six cycles of bendamustine (70 mg/m2) in combination with rituximab (375 mg/m2), and the patient is currently complete remission state. Waldenström’s macroglobulinemia (WM), a type of LPL, is characterized by the presence of IgM monoclonal protein [1]. Light chain cast nephropathy is a form of acute or chronic kidney disease that arises from the excessive production and filtration of toxic light chains [2]. While renal biopsy-confirmed cases of cast nephropathy are commonly linked to multiple myeloma, their occurrence in WM is comparatively infrequent [3].

This case highlights the necessity of a comprehensive diagnostic approach in patients with a history of diabetes and hypothyroidism, emphasizing the need to look beyond diabetic nephropathy. Additionally, it illuminates the possibility of cast nephropathy manifesting in LPL, expanding its known association beyond multiple myeloma.