CK, or creatinine kinase, is an enzyme responsible for the cells’ energy production. There are three isoenzymes (types):
- CK-MM: located in skeletal and cardiac muscle
- CK-BB: mostly present in the brain and smooth muscle (e.g. uterus)
- CK-MB: found mainly in the myocardium, but also in smaller amounts in the skeletal muscle
By determining the level of the CK-MB enzyme, myocardial infarction and other myocardial damage can be diagnosed, and skeletal and myocardial damage can be distinguished, as in case of muscle damage, CK enters the bloodstream from the muscle cells, where CK-MM present in small amounts (BB almost never). As a result, if CK levels are elevated, the CK-MB test helps differentiate whether the increase is caused by heart or skeletal muscle damage. CK-MB typically appears in the bloodstream only when there is damage to the heart muscle in the body.
However, it is worth noting that nowadays, measurement of troponin levels is used more to detect and monitor myocardial damage.