Methods of using biologically active dimerized polypeptide fusions to detect PDGF

A method for determining the presence of platelet-derived growth factor (PDGF) or isoforms thereof in a biological sample, comprising:

• incubating a multimeric polypeptide comprising two polypeptide fusions, each of said polypeptide fusions comprising a ligand binding domain and a dimerizing domain, wherein said ligand binding domain corresponds to a ligand binding domain of a PDGF receptor, wherein the receptor is a homodimer composed of subunits, each of which has a single extracellular ligand binding domain, a single transmembrane domain and a single cytoplasmic domain, and wherein said dimerizing domain is a polypeptide chain having affinity for a second polypeptide chain such that the two chains associate under physiological conditions to form a dimer, with a biological sample suspected of comprising PDGF or an isoform thereof under physiological conditions to allow the formation of receptor/ligand complexes;
• and
• detecting the presence of the receptor/ligand complexes as an indication of the presence of human PDGF or an isoform thereof.