The present invention provides a method for the quantification of NO in a blood sample comprising the steps of collecting a blood sample, dividing said collected blood sample into three blood subsamples, chemically treating one blood subsample with an antioxidant to at least partly reduce free radicals of said blood subsample, performing EPR measurement of said blood subsamples, performing a first comparison of said chemically treated blood subsample EPR measurement with the EPR measurement of a non-chemically treated blood sub-sample thereby obtaining a first comparison result and performing a second comparison of this result with an EPR measurement of the remaining blood subsample in order to eliminate background signals.