The invention provides, in certain embodiments, a method of assaying an indicator of, and treating, renal injury or renal disease. The method entails assaying a urine sample for periostin, wherein the presence of periostin at an elevated level indicates the presence and/or degree of renal injury or renal disease. The method also involves prescribing, initiating, or altering prophylaxis or therapy for renal injury or renal disease. Also provided, are methods of determining progression of these conditions, as well as methods of determining subjects' response to treatment.