The present invention relates to a method for differentially detecting a multimeric form from a monomeric form of a multimer-forming polypeptide in a biosample, which comprises the steps of: (a) preparing a carrier-capturing antibody conjugate by binding a capturing antibody to the surface of a solid phase carrier in a three dimensional manner, wherein the capturing antibody is capable of recognizing an epitope on the multimer-forming polypeptide; (b) preparing a detection antibody, wherein an epitope recognized by the detection antibody is present at a position in the multimer-forming polypeptide to cause a steric hindrance by the capturing antibody bound to its epitope to prevent the binding of the detection antibody to the multimer-forming polypeptide; (c) contacting simultaneously the carrier-capturing antibody conjugate and the detection antibody to the biosample; and (d) detecting the formation of a carrier-capturing antibody-multimeric form-detection antibody complex.