The present invention concerns in general the field of antigen-antibody-interaction-based analysis-methods and kits therefore. In particular, the present invention concerns a method for a quantitative in vitro analysis to diagnose, to categorise, to predict and/or to monitor the progression of a condition comprising contacting a sample suspected of containing anti-A-antibodies from a subject to be analysed with native and mutant antigen A, wherein the binding to mutant antigen A helps to eliminate unspecific binding to antigen A and furthermore helps to eliminate individual patient based artefacts, so that clear and unbiased data for the formation of complexes of anti-A-antibodies and antigen A are obtained. The present invention also relates to a kit for carrying out such a method. For example, the antigen is the MOG protein, and it is used as a marker for Multiple Sclerosis.