Creatinine is a waste product formed from the normal breakdown of muscle tissue. It is produced at a relatively constant rate, depending on muscle mass, and is filtered out of the blood by the kidneys. Measuring the level of creatinine in the blood or urine helps assess kidney function. Elevated levels may indicate that the kidneys are not working properly, while normal levels suggest that they are filtering waste effectively. Regular testing can help monitor kidney health and detect potential issues early.