Creatinine

n. chemical compound found in the muscles of vertebrate animals

Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass).
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Creatinine /cre·at·i·nine/ an anhydride of creatine, the end product of phosphocreatine metabolism; measurements of its rate of urinary excretion [more]Creatinine - Community and Resources

[cree-AT-ih-nin]
A compound that is excreted from the body in urine. Creatinine levels are measured to monitor kidney function. 

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.