This test measures how well your kidneys are performing their job of filtering waste from your blood. Creatinine is a chemical compound left over from energy-producing processes in your muscles. Healthy kidneys filter creatinine out of the blood. Creatinine exits your body as a waste product in urine

During the metabolism of protein in the body, the liver creates ammonia, which is broken down into a by-product called urea. Kidneys filter excess urea into the urine and in sweat, but some goes into the bloodstream as serum urea.