Kidney Week

Abstract: FR-PO1289

Fanconi Syndrome and Osteomalacia as the Initial Presentation of Light-Chain Proximal Tubulopathy Associated with Smoldering Multiple Myeloma

Session Information

• Onconephrology: Diagnostic Dilemmas, Therapy-Related Toxicities, and Clinical Cases
  October 23, 2026 | Location: Exhibit Hall A, Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Onconephrology

• 1600 Onconephrology

Authors

• Roberto, Lucas Enock Vieira, Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Lucas Enock Vieira Roberto, MD

• Almeida, Vívian Cristine Lima de, Universidade Federal de Sao Paulo Escola Paulista de Medicina, São Paulo, SP, Brazil

Vívian Cristine Lima de Almeida, MD

• Moyses, Rosa M.A., Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Rosa M.A. Moyses, MD, PhD

• Mattedi, Francisco Zanotelli, Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Francisco Zanotelli Mattedi, MD

• Segura, Gabriela C., Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Gabriela C. Segura, MD

• Kassar, Liliana M L, Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Liliana M L Kassar, MD

• Lutf, Luciana Gil, Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Luciana Gil Lutf, MD

• Caires, Renato A., Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Renato A. Caires, MD

• Costa e Silva, Veronica Torres, Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo, São Paulo, SP, Brazil

Veronica Torres Costa e Silva, MD, PhD

Introduction

Light Chain Proximal Tubulopathy (LCPT) is an uncommon manifestation of monoclonal gammopathies but can be associated with high morbidity due to osteomalacia secondary to chronic hypophosphatemia.

Case Description

A 41-year-old woman presented in February 2025 with weight loss, bone pain, and weakness. Metabolic investigation revealed Fanconi syndrome: normoglycemic glucosuria, hypophosphatemia, hypouricemia, hypokalemia, and hyperchloremic metabolic acidosis, associated with hyperphosphaturia, hyperuricosuria, and tubular proteinuria (Table). Also, the patient presented high alkaline phosphatase levels and low bone mineral density on densitometry, which, combined with bone pain, were highly suggestive of osteomalacia. Further investigation confirmed the diagnosis of smoldering multiple myeloma (MM) with monotypic kappa plasmocytes in bone marrow biopsy (Table). The constellation of findings was consistent with LCPT. Considering the severity of electrolyte disorders with clinical implications, the patient was treated with bortezomib, cyclophosphamide, and dexamethasone, achieving a complete hematologic response after 6 cycles; however, she remained dependent on high-dose electrolyte replacement and calcitriol due to the chronicity of proximal tubular injury.

Discussion

This case highlights the importance of investigating monoclonal gammopathy in patients with hypophosphatemia and osteomalacia. Although chemotherapy allows a sustained hematologic response, recovery of tubular function is often incomplete, requiring prolonged metabolic support.

Laboratory tests