A diagnostic method for inflammatory diseases, and the diagnostic kit used in the method. The diagnostic method includes the use of a novel tryptophan metabolite, 5-methoxytryptophan (5-MTP), as a diagnostic biomarker of inflammation. Specifically, a highly specific competitive ELISA is provided to measure 5-MTP level in human serum for gauging occurrence and severity of inflammatory diseases, including sepsis and systemic lupus.
