Kidneys are responsible for filtering blood in the human body. The substance creatine is formed during metabolism. Creatine is then broken down into another substance called creatinine, which is taken out of your blood by the kidneys and then passed out of your body in urine, among other substances like urea nitrogen. If your kidneys are damaged and aren’t functioning normally, the amount of these wastes in your blood is high. Apart from malfunctioning kidneys, heart failure, dehydration, certain medicines, intestinal bleeding, or a diet high in protein also can increase your Blood Urea Nitrogen (BUN) level. BUN test checks your blood for sign of damaged kidneys. A creatinine clearance test is done on both a blood sample and on a sample of urine collected over 24 hours.