CK is an 85-kDa enzyme found in all striated muscle cells, where it catalyzes the phosphorylation of creatine to creatine phosphate. There are three isoenzymes of CK, each composed of two subunits (M and B): CK-MM, the predominant form in striated muscle (cardiac and skeletal); CK-MB, most common in the heart; and CK-BB, most common in the brain but also found in the gut and kidney. Sensitivity for acute MI is greater than 90% by 6 h, and total CK may be detectable in the serum as early as 4 h after the onset of symptoms. Because assays for total CK detect all three isoenzymes, however, total CK is not cardiac-specific.
The reference range for total CK assays depends on the patient's age, sex, and race, and levels of total CK may be falsely elevated in many pathological and other conditions (13,53). Therefore, it is a relatively nonspecific marker that alone has limited utility for the diagnosis of myocardial injury.
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