Creatinine is the chemical waste product of creatine phosphate in the muscles; it is produced by the body at a fairly constant rate during regular muscle break down. Each day, the body converts about 2 percent of its creatine to creatinine, according to MedicineNet.
The amount of creatinine in the blood can be measured via a creatinine blood test, which is performed to determine whether the kidneys are functioning properly. Alternatively, a urine test can also be used to measure creatinine. The kidneys are responsible for filtering creatinine from the blood. The creatinine clearance rate refers to a kidney's ability to handle creatinine.