Investigating Denosumab Interference in Serum Protein Electrophoresis

Therapeutic monoclonal antibodies, widely used in multiple myeloma (MM), can interfere with the interpretation of serum protein electrophoresis (SPEP) and immunofixation electrophoresis results, challenging the differentiation from endogenous monoclonal (M) proteins associated with the disease. Although this interference has been well established for several therapeutic monoclonal antibodies, the potential impact of denosumab, an IgG2(κ) antibody used to treat MM-related bone disease, remains unclear.

We characterized the electrophoretic migration of denosumab using in vitro spiking experiments. To evaluate in vivo interference, we performed a retrospective review of SPEP and immunotyping results from 110 patients with MM receiving denosumab. Finally, we used matrix-assisted desorption/ionization time-of-flight mass spectrometry (using the EXENT system) to distinguish between denosumab and endogenous M-proteins based on molecular mass.

In vitro, denosumab migrated to the mid-to-distal gamma region, nearer the cathodal end than daratumumab, with a normalized migration ratio of approximately 1.34. Among the 110 patients, one exhibited a corresponding IgG(κ) peak on SPEP and immunotyping with the specified migration position, although mass spectrometry confirmed this to be an endogenous M-protein rather than denosumab. No denosumab-specific interference was detected above the lower limit of quantification (15 mg/L) using mass spectrometry.

Although denosumab exhibited a consistent electrophoretic migration pattern in vitro, it did not cause clinically relevant in vivo interference in SPEP or immunotyping assays. These findings support accurate electrophoretic interpretation and reduce concerns regarding misclassification of M-proteins in patients with MM receiving denosumab.