The present invention provides a method of effectively measuring risk for therapeutic toxicity of a subject having an autoimmune disorder or cancer and predicting and evaluating therapeutic efficacy of immunosuppressive therapies for autoimmune diseases and cancers before or after starting therapy. The present invention also provides for determining a drug metabolite level of a subject during therapy and measuring periodically the drug metabolite level of a subject on maintenance therapy to ensure treatment compliance and continued therapeutic response by measuring minimal clinical important differences (MCID) in the drug metabolite levels. The present invention also provides for a method to effectively optimize the selection and dose of immunosuppressive therapies of a subject having an autoimmune disease or cancer to improve therapeutic efficacy and reduce therapeutic toxicity prior to starting concomitant biologic therapy and before or after the subject has failed to respond to the at least one immunosuppressive agent.