Provided herein is a sandwich immunoassay for detecting cross-linked PIIINP that has at least two strands of PIIINP joined together by inter-strand cross-linking each having a C-terminal neo-epitope of PIIINP that is generated by N-protease cleavage of intact type III procollagen. A biological sample having the cross-linked PIIINP is contacted with a first surface-bound monoclonal antibody and then by a second monoclonal antibody, both specifically reactive with a neoepitope in the C-terminal sequence of PIIINP, and then binding of the second monoclonal antibody is determined. Also provided is a method for evaluating the efficacy of an antagonist drug targeting lysyl oxidases via the immunoassay and a kit containing a solid support binding the first monoclonal antibody and containing the second monoclonal antibody.