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
