Creatine kinase (CK) is an enzyme found in high concentrations in muscle cells, including the heart. It catalyses the conversion of creatine and ATP to phosphocreatine, providing rapid energy for muscle contraction. When muscle cells are damaged, CK leaks into the bloodstream and its level rises proportionally to the degree of injury.

CK is used to investigate conditions causing muscle damage including rhabdomyolysis, myopathies, muscular dystrophies, inflammatory muscle disease, and statin-associated muscle toxicity. CK-MB, a cardiac-specific isoenzyme, rises in myocardial infarction — though troponin has largely superseded CK-MB for cardiac diagnosis. Very high CK from skeletal muscle breakdown can cause acute kidney injury due to myoglobin release.