Resolving Therapeutic Monoclonal Antibody Interference in an Immunoglobulin M Kappa Multiple Myeloma Patient Using a Mass Spectrometry Assay

BACKGROUND: Therapeutic monoclonal antibodies can interfere with immunofixation electrophoresis (IFE) during multiple myeloma monitoring. CASE PRESENTATION: Serum samples from an immunoglobulin M kappa (IgMκ) myeloma patient on daratumumab therapy were analyzed by serum protein electrophoresis (SPEP)/IFE and a mass spectrometry assay. A new immunoglobulin G kappa (IgGκ) band appeared on IFE during active treatment. Mass spectrometry identified a monoclonal peak at m/z 23,386, matching daratumumab. The band disappeared at trough concentration. CONCLUSIONS: Mass spectrometry rapidly identified daratumumab interference, serving as a reagent independent confirmatory tool to avoid misinterpreting therapeutic antibodies as clonal evolution.