Herein is reported a method for the detection of free antigen of a multispecific antibody in a sample, whereby the antigen to be detected can be specifically bound by a first binding site of the multispecific antibody, comprising the step of incubating a sample comprising free antigen and multispecific antibody with an anti-idiotypic antibody, which specifically binds to a second binding specificity of the bispecific antibody, which is different from the first binding specificity, whereby the anti-idiotypic antibody is bound to a solid phase.