Creatinine is a breakdown product of creatine, which is an important constituent of muscle. Creatinine can be converted to the ATP molecule, which is a high-energy source. The daily production of creatine and subsequently creatinine, depends on muscle mass, which fluctuates very little.
Creatinine is excreted from the body entirely by the kidneys. With normal renal excretory function, the serum creatinine level should remain constant and normal.
A normal (usual) value is 0.8 to 1.4 mg/dl.
Females have a lower creatinine than males, due to decreased muscle mass.
Note: mg/dl = milligrams per deciliter