a waste product from protein in the diet and from the muscles of the body. Creatinine is removed from the body by the kidneys; as kidney disease progresses, the level of creatinine in the blood increases. www.diabetes.org. a substance in the blood (a piece of a protein) which should be at a constant level in the blood. When creatinine levels rise in the blood, it can be a sign that the kidneys are not functioning well. Sometimes, creatinine is measured in the blood and in the urine, as part of a Creatinine Clearance test. www.abcbirth.com