Creatine kinase (CK), a dimeric protein, is an enzyme that catalyzes the transfer of high-energy phosphate from ATP to creatine and thus is important in maintaining cellular energetics and muscle cell metabolism. CK is not unique to skeletal muscle. The highest tissue concentrations of CK are found in cardiac muscle, with lesser amounts in skeletal muscle and other tissues. Three different isoenzyme forms of CK exist in various tissues (CK-MM, CK-MB, and CK-BB). CK-MM is the predominant isoenzyme in skeletal muscle, where it is found in the myofibril bound to the M-line structure of the sarcomere. In normal serum, the level of total CK is primarily of skeletal muscle origin and is mainly the MM fraction (94–100%).

CK-MM levels increase in response to skeletal and/or cardiac tissue injury. Among the various enzyme biomarkers, CK appears to be the most sensitive. In some instances, CK may be the only enzyme that is elevated. As a result, monitoring changes in serum CK concentrations has been a widely utilized biochemical means to detect the presence of myofibrillar injury. Serum concentrations of CK begin to increase within 2–12 hours after the onset of muscle injury, reach a maximum at 24–72 hours, and drop at 3–5 days as the muscle injury subsides. The maximum levels of serum CK attained vary according to the type of myopathy and the progression of injury.For example, a CK concentration that is chronically elevated tends toindicate the occurrence of ongoing muscle injury. Injury involving 200 grams of muscle is sufficient to cause an increase in serum CK. Clinically; certain increases in serum levels of CK have been suggested to define conditions of severe muscle injury (greater than 10 times the upper limit of normal in the case of statin-induced rhabdomyolysis).