Creatinine is a chemical compound left over from energy producing processes in your muscles. Healthy kidneys filter creatinine out of the blood & it exits your body as a waste product in urine. Your doctor prescribes this test if you have signs or symptoms of kidney disease; to screen for kidney disease if you have diabetes, high blood pressure or other conditions that increase the risk of kidney disease; to monitor kidney disease treatment; to monitor for side effects of drugs that may affect kidney function & to monitor if you have had a kidney transplant.