Methods are described for improving the diagnostic possibilities of diseases where oxidative NO-modifications occur, for example inflammatory conditions, cancer, Parkinson's or Alzheimer's disease, and to provide means of monitoring the effects of therapeutical measures taken towards such diseases. The invention enables the detection of disease specific catabolic markers related to oxidative NO-modifications, utilizing an immunoassay comprising antibodies directed against nitrated and non-nitrated epitopes characteristic of a specific protein.