Light-chain multiple myeloma (LCMM) accounts for a subset of myeloma cases characterized by the absence of an M-protein spike on serum protein electrophoresis. This diagnostic challenge often delays recognition and treatment. We present the case of a 75-year-old woman with kappa-dominant LCMM, where conventional marrow and serum studies were inconclusive, yet clinical and imaging findings confirmed active disease.

A comprehensive diagnostic evaluation was performed, including hematology and biochemistry profiles, serum protein electrophoresis, serum and urine immunofixation, serum free light chain (sFLC) quantification, bone marrow aspiration and biopsy with immunohistochemistry, and 18F-FDG PET-CT imaging.

Gülüşen Kellesibüyük, a 75-year-old female, presented with fatigue, anemia, and back pain. Laboratory evaluation revealed hemoglobin of 9.7 g/dL, elevated inflammatory markers, and preserved renal and calcium levels. Serum protein electrophoresis demonstrated no monoclonal spike. Immunofixation of urine identified monoclonal kappa light chains. sFLC testing showed markedly increased kappa levels (121–270 mg/L) with a pathological κ/λ ratio between 3.9 and 4.2. Bone marrow aspirates revealed only 2–3% plasma cells with polytypic staining, and biopsies were normocellular without evidence of clonal infiltration. Despite these inconclusive marrow results, PET-CT demonstrated a metabolically active lytic lesion in the L4 vertebra (SUVmax 11.4) and multiple punctate cranial lytic lesions. The combination of anemia, abnormal light chain ratio, and PET-CT–confirmed bone lesions established the diagnosis of active LCMM.

This case emphasizes the diagnostic complexity of LCMM, where reliance solely on serum electrophoresis or marrow histology may be misleading. The absence of an M spike, coupled with non-diagnostic marrow sampling, initially obscured the diagnosis. However, integration of sFLC analysis, urine immunofixation, and advanced imaging confirmed the presence of active myeloma. Elderly, transplant-ineligible patients such as this one benefit from modern therapeutic approaches that combine efficacy with tolerability. Triplet regimens including daratumumab with lenalidomide and dexamethasone or reduced-intensity bortezomib-based protocols are recommended as first-line options. For patients with limited access to hospital care, oral regimens may be considered, though efficacy is comparatively lower, Türkiye.

The case of demonstrates that light-chain multiple myeloma can be present despite normal serum electrophoresis and non-clonal marrow findings. Comprehensive evaluation with free light chain assays, urine studies, and PET-CT is essential to avoid underdiagnosis. This case highlights the importance of applying full diagnostic criteria to detect atypical myeloma presentations early, ensuring timely initiation of therapy and improved patient outcomes.