A substance that comes from muscle metabolism and is filtered by the kidneys at a rate that is usually constant in healthy people. In some cases, both the underlying disease and medications can alter this rate. Since creatinine can easily be measured in blood and urine, a creatinine level is a good indicator of how well the kidneys are functioning. It should be noted that changes in creatinine levels in the blood and urine, as well as changes in the filtration rate, can be found in a variety of situations, many of which have nothing to do with cancer.
