Interference-suppressed pharmacokinetic immunoassay

A method for the determination of a bispecific antibody in a serum-containing sample comprising the following steps:

• a) incubating a solid phase with the serum-containing sample,
• wherein the serum-containing sample comprises the bispecific antibody and a monospecific antibody,
• wherein the bispecific antibody comprises a first binding site that binds to a first antigen and a second binding site that binds to a second antigen, and wherein the monospecific antibody comprises the second binding site,
• wherein the solid phase comprises a capture antibody immobilized thereon,
• wherein the capture antibody specifically binds the first binding site to form an immobilized capture antibody-bispecific antibody-complex, and wherein the monospecific antibody nonspecifically binds the solid phase,
• b) optionally washing the solid phase whereby the capture antibody-bispecific antibody-complex remains bound to the solid phase,
• c) incubating the solid phase with a solution comprising a replacement antibody that competes with the capture antibody for binding to the first binding site of the bispecific antibody, thereby forming a replacement antibody-bispecific antibody-complex in the solution, wherein the monospecific antibody remains bound to the solid phase, and
• d) determining the replacement antibody-bispecific antibody-complex in the solution of step c) and thereby determining the bispecific antibody in a serum-containing sample.