Creatinine is a byproduct of creatine produced in the muscles and is removed from the body by the kidneys. Measuring the amount of creatinine in the blood and/or urine can predict how well the kidneys are working. Immunoassay is a general method of detecting creatinine. However, creatinine is a very small molecule and is generally very difficult to generate an antibody against and that binds strongly to the protein for detection.