Unmasking amyloid light-chain amyloidosis through biochemical lens: diagnostic utility of urine immunofixation in serum-negative cases

Abstract Objectives AL amyloidosis is a rare disorder caused by deposition of misfolded immunoglobulin light chains as amyloid fibrils in vital body organs. The diagnosis requires a triad: identification of a monoclonal protein (via serum/urine studies), histological confirmation of amyloid deposits and clinical evidence of organ dysfunction. Serum protein electrophoresis (SPEP) and immunofixation (SIFE) are first-line tests but fail to detect monoclonal proteins in 10–20 % of cases, particularly those with low plasma cell burden or rapid renal excretion of FLCs. Serum free light chain (FLC) assays and urine immunofixation (UIFE) are indispensable in such scenarios but tissue biopsy remains the diagnostic cornerstone. Case presentation We discuss a 67-year-old man who presented with a 4-month history of progressive bilateral lower limb edema, fatigue and frothy urine. Initial evaluation revealed nephrotic-range proteinuria. SPEP and SIFE showed no monoclonal bands. X-ray skull revealed multiple punched-out lytic lesions, raising suspicion of an underlying plasma cell dyscrasia. UIFE identified a monoclonal lambda light chain. Renal biopsy confirmed amyloid deposition. The patient was initiated on bortezomib-dexamethasone chemotherapy, targeting the plasma cell clone to halt amyloid production. Conclusions This case underscores the diagnostic challenges of AL amyloidosis, particularly in serum-negative presentations with low tumor burden. Role of UIFE was pivotal in detecting monoclonal lambda light chains excreted via the kidneys, overcoming the limitations of serum-based assays. The absence of serum monoclonal proteins in early-stage disease mandates a multimodal approach: integrating clinical suspicion (e.g., nephrotic syndrome, cardiomyopathy, or neuropathy in older adults), urine studies, serum FLC assays, and targeted biopsies.