Evusheld, a SARS-CoV-2 spike protein-directed attachment inhibitor, appears in serum protein electrophoresis and...

Serum protein electrophoresis (SPE) and immunofixation (IFE) assays are commonly used to diagnose and monitor patients with multiple myeloma (MM). Identifying analytical interferences in SPE and IFE caused by therapeutic monoclonal antibodies (tmAbs) can be challenging. Here we report the case of a 72-year-old male with a long history of relapsed immunoglobulin (Ig)G kappa MM. A follow-up SPE showed the original peak plus 2 additional cathode peaks. Immunofixation was ordered as a reflex test to investigate the new peaks that showed initial patient monoclonal IgG kappa in addition to 2 restricted bands of the IgG kappa type. Therapeutic monoclonal antibody interference was suspected and the patient's chart was reviewed. The patient was not on any antimyeloma monoclonal antibody therapy. However, preexposure prophylaxis therapeutic monoclonal antibodies tixagevimab plus cilgavimab (Evusheld) for severe acute SARS-CoV-2 was administered approximately 45 minutes before sample collection, which led to the identifiable spikes and correlated bands. After 2 days, the IgG kappa bands disappeared, confirming this therapy's effect on SPE and IFE. Therefore, clinical pathologists should be aware of when providers prescribe new monoclonal antibody therapy and become familiar with the position of commonly prescribed (tmAbs) therapies at their institutions.