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to diagnosis of acute myocardial infarction by lactate dehydrogenase (LD) isoenzymes. However, increases of LD isoenzyme activities are not specific for myocardial injury. Recently, increased concentrations of cardiac troponin I (cTnI) have been shown to be highly specific for myocardial damage and to have sensitivity comparable with that of creatine kinase MB isoenzyme for detecting cardiac injury. Furthermore, increases of cTnI persist in plasma for at least several days. The present study was designed to determine the relative sensitivities of cTnI and LD isoenzymes over time for the diagnosis of infarction. The results indicate that cTnI values are at least as sensitive as LD isoenzymes:90% of patients with myocardial infarction had above-normal concentrations of cTnI on the4th day after admission to the coronary care unit. Criteria based on cTnI should improve the accuracy of retrospective diagnoses of acute myocardial infarction.
The MB isoenzyme of creatinine phosphokinase (CPK) has been described as a specific indicator of myocardial injury. CK is a dimer composed of two different subunits, B and M. The three isoenzymes are designated MM, MB and BB. The CK isoenzymes are resolved by electrophoresis but many hospital laboratories conduct CK isoenzyme analysis by a simple procedure involving the use of an antibody against the M subunit which inhibits M activity while the activity of the B subunit is retained. The BB isoenzyme is present in brain and intestinal smooth muscle, but the BB isoenzyme is rarely present in serum, even in cases of nervous system or intestinal disease. The major CK isoenzyme of both skeletal muscle and myocardium is MM, and both tissues contain the MB isoenzyme. The ratio of MB/MM is, however, different for each tissue. The skeletal muscle content of MB is less than1% of total CPK activity, whereas the myocardial content of MB is about25-50% of total CK activity. Normal serum activity is almost entirely from the MM isoenzyme and the MB content of normal serum is usually undetectable. Degenerative muscle disease or crushing muscle injury results in elevated total serum CK activity, but the MB isoenzyme content is less than5%. Serum MB activity greater than5% of total is considered diagnostic for MI.
Troponin and ckmb isoinzaym
Lactate dehydrogenase (LD) isoenzymes: sensitive but not specific for myocardial injury.
Cardiac troponin I (cTnI) : highly specific for myocardial damage and sensitive & persist in plasma for at least several days so may be used as follow up to patient condition.
Creatine kinase MB isoenzyme: Sensitive & has types may be used as a ratio to confirm diagnosis .
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