In the bloodstream, CK-MB exists predominantly in equilibrium between two forms, the tissue form (MB2) and the circulating seroconverted form (MB1). As cardiac muscle cells die, the MB2 subform is released and converted to MBlin the serum by car-boxypeptidase cleavage of the N-terminal lysine on the M subunit. During an acute MI, large amounts of MB2 are released, increasing the ratio of MB2 to MB1 as well as the absolute amount of circulating MB2. Puleo and colleagues have shown that an absolute level of MB2 > 1 U/L and an MB2/MB1 ratio of >1.5 are very sensitive markers of myocardial necrosis (33). Using the CK-MB subform assay, myocardial necrosis can be detected as early as 3 h after the onset of ischemic symptoms. The use of the MB subform assay has the same limitations in specificity as does CK-MB, but it has excellent sensitivity and negative predictive value in patients presenting with chest pain.
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