Doctors sometimes use the enzyme creatine phosphokinase, found mostly in the heart, brain, and muscles, as an indicator of injury to one or more of these parts of the body.
CPK's normal function is the transformation of creatine acid into phosphate, which is a usable source of energy for muscle, heart, and brain cells.
The normal concentration of CPK in the blood of a healthy adult is 22 to 198 units per liter. An unusually high concentration of CPK may indicate an injury or illness.
When an organ or muscle containing CPK is damaged, the bloodstream floods with the "spilled" enzyme. Analyzing CPK levels through a blood test enables doctors to find out exactly what kind of CPK it is, thus revealing where the damage lies.
In the liver, the most common explanation for raised CPK is rhabdomyolysis, a common but dangerous skeletal muscle disorder that releases toxins from the muscle cells into the bloodstream.
Fortunately, disease or injury isn't the only explanation for high CPK levels. Vigorous exercise can flood the blood with CPK, as can surgical procedures or even falls. Nevertheless, CPK tests are useful, particularly in differential diagnoses--the systematic elimination of possibilities through a series of tests.