Fusion proteins comprising an extracellular domain of PD1 (programmed cell death protein-1) protein and/or an extracellular domain of PD-L1 (programmed cell death-ligand 1 protein (CD274 or B7-H1)) protein. Portions of the extracellular domains are expressed in specific configurations and purified as protein and used in immunoassays to monitor the circulating levels of biotherapeutic antibodies to these proteins. Also described is a method of determining the amount of circulating levels of a biotherapeutic antibody in a biological sample obtained from a patient, wherein a patient has undergone at least one dose of immunotherapy.