Elotuzumab is a first in class humanized IgG1 monoclonal antibody for the treatment of multiple myeloma (MM). Elotuzumab targets the glycoprotein signaling lymphocyte activation molecule family 7 (SLAMF7, also described as CS1 or CRACC) which is expressed on the surface of myeloma cells and a subset of immune cells, including natural killer cells. A soluble version of SLAMF7 (sSLAMF7) has also been reported in MM patients but has not been evaluated as a potential biomarker following therapeutic intervention. In order to measure serum levels of sSLAMF7, two immunoassays were developed to monitor changes in circulating sSLAMF7 before and after elotuzumab treatment. Free (drug-unbound) and total (drug-bound and unbound) electrochemiluminescence (ECL) ELISA assays were developed and validated following a fit for purpose (FFP) methodology. Both assays met analytical acceptance criteria for precision, drug interference, dilution linearity, spike recovery, parallelism, and stability. Both exhibited the range and sensitivity necessary to measure clinical samples with an LLOQ of 51.2 pg/mL and ULOQs of 160 (free) and 800 ng/mL (total). Previously described assays were unable to detect sSLAMF7 in healthy individuals. However, due to the increased sensitivity of these new assays, low but measurable sSLAMF7 levels were detected in all normal healthy sera evaluated and were significantly elevated in MM patients. Cohort statistics revealed a significant increase of circulating sSLAMF7 in MM patients versus normal controls and both significant decreases in free and increases in total levels of protein post-elotuzumab treatment.