120816: Creatine Kinase (CK), MB

Creatine kinase (CK) is an enzyme, found primarily in muscle and brain tissue, which exists as three dimeric isoenzymes: CK-MM (CK-3), CK-MB (CK-2), and CK-BB (CK-1) − built from subunits designated M and B. The CK-MB isoenzyme, which has a molecular mass of approximately 87 kilodaltons, accounts for 5% to 50% of total CK activity in myocardium. In skeletal muscle, by contrast, it normally accounts for ≤1%, CK-MM being the dominant form, though the percentage can be as high as 10% in conditions reflecting skeletal muscle injury and regeneration (eg, severe exercise, muscular dystrophy, polymyositis).1

CK-MB is one of the most important myocardial markers (in spite of not being altogether cardiac-specific), with well-established roles in confirming acute myocardial infarction (AMI) and in monitoring reperfusion during thrombolytic therapy following AMI.1

In AMI, plasma CK-MB typically rises some four to six hours after the onset of chest pains, peaks within 12 to 24 hours, and returns to baseline levels within 24 to 48 hours. The pattern of serial CK-MB determinations is more informative than a single determination: one CK-MB measurement, even when taken at an appropriate time, cannot definitively confirm or rule out the occurrence of AMI. High levels might reflect skeletal injury rather than myocardial damage. A value within the reference range might be significant if it represents an increase from the patient's baseline level. (Low baseline levels are sometimes encountered in the elderly.) Accordingly, it has been recommended that CK-MB be measured on admission to the emergency room and at intervals thereafter (eg, at three-hour intervals over a six-hour to nine-hour period in patients with nonspecific electrocardiogram changes;1,2 or at six-hour to eight-hour intervals over a 24-hour period and more frequently if thrombolytic therapy has been instituted).1