Although CK-MM is the predominant isoenzyme of CK in both cardiac and skeletal muscle, CK-MB is relatively cardiac-specific, with only small amounts (up to 5%) detected by immunoassay in skeletal muscle. However, CK-MB may be produced in increased amounts in skeletal muscle after trauma or in inflammatory conditions.
Because the CK-MB isoform is found predominantly in cardiac muscle, it is more specific for the diagnosis of myocardial injury and, therefore, provides a distinct advantage over total CK. It becomes elevated within 3 to 4 h after the onset of ischemic myocardial injury and returns to baseline within 24-36 h. Therefore, although it provides a nearly ideal timing profile for a marker of major myocardial necrosis, it remains limited in its ability to detect small amounts of myocardial damage (infarctlets) that may be prognostically important but obscured by background release from normal turnover of skeletal muscle cells.
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