Elotuzumab can be detected in SPEP and IFE in samples from patients treated with elotuzumab. (A) Serum IFE detected an IgA-λ M-protein at baseline. (B) After 8 cycles of lenalidomide-dexamethasone and elotuzumab (ELOQUENT-2 study), there was development of a new IgG-κ band of 0 to 2 g/L. In the anti-elotuzumab assay, one of the anti-immunoglobulin antibodies (anti-IgM or anti-IgA) used to precipitate the immunoglobulins was replaced by an anti-elotuzumab antibody (2 mg/mL) with reactivity to an elotuzumab epitope. If elotuzumab is present in patient sera, the anti-idiotypic antibody-elotuzumab complex precipitates, and a band is detected. As elotuzumab is an IgG-κ monoclonal antibody, elotuzumab is precipitated by anti-κ and anti-γ heavy chain antisera, and the 2 bands should align with the elotuzumab band. Indeed, the anti-elotuzumab assay, performed at cycle 34, detected elotuzumab in the IgG-κ band. SIFE, serum immune fixation.