CK-MB is a specific isoenzyme of creatine kinase (CK) mainly present in cardiac muscle, although it is also found in lesser quantities in skeletal muscle.

CK-MB has historically been used as one of the main markers for diagnosing myocardial infarction. In the event of myocardial damage, such as a heart attack, damaged heart cells release CK-MB into the bloodstream. A significant rise in

CK-MB is therefore indicative of cardiac damage.

CK-MB begins to rise in the blood around 4 to 6 hours after the onset of a heart attack, peaks at 12 to 24 hours and returns to normal within 48 to 72 hours. These kinetics are useful for diagnosing recent infarctions or for detecting infarct recurrences.

When total CK is elevated, the CK-MB assay can be used to determine whether the elevation originates mainly from the heart or skeletal muscle. A high proportion of CK-MB (relative to total CK) suggests a cardiac origin.

When a patient presents with chest pain, the CK-MB assay, although largely replaced by cardiac troponins, may still be used in some contexts to help differentiate a cardiac cause from chest pain of non-cardiac origin.