Creatine kinase (CK)

Creatine kinase (CK) is an enzyme that catalyses the reversible transfer of a phosphate group from phosphocreatine to ADP, regenerating ATP in tissues with high and fluctuating energy demands, principally skeletal muscle, cardiac muscle, and brain. Serum CK is released when these tissues are damaged, with isoenzyme fractionation — CK-MM (skeletal), CK-MB (cardiac), CK-BB (brain) — guiding organ-level attribution; historically CK-MB elevation was a diagnostic criterion for myocardial infarction, but since the 2018 Fourth Universal Definition of MI cardiac troponin (preferably high-sensitivity) has replaced CK-MB as the standard biomarker. Transient CK elevations after unaccustomed strenuous exercise are expected and benign, whereas persistent elevation suggests myopathy, rhabdomyolysis, or statin-induced muscle toxicity. In sarcopenia research, resting CK and its trajectory are explored as indirect indices of muscle membrane integrity.