Measurements of Enzymes in the Diagnosis of Acute Myocardial Infarction

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Abstract
The determination of the levels of activity of various enzymes in the blood or serum of man represents a major advance in the laboratory diagnosis of various disease states. The widespread body distribution of various enzymes at relatively large tissue activity levels in comparison to the small amounts normally in the circulation has, on the one hand, permitted the demonstration of diseased or damaged tissue by the resultant elevation of serum levels of activity. On the other hand, the generally large amounts of the clinically significant enzymes in heart, liver, skeletal muscle, kidney, brain, etc. has precluded the desired specificity of a laboratory test. At the present state of development, increases in circulating levels of various enzymes appear to be more sensitive indicators of tissue damage than previously available ones, such as the erythrocyte sedimentation rate and the C-reactive protein but it must be remembered that they are equally nonspecific. Any cause of tissue breakdown—trauma, infection, infarction, ischemia, neoplasia—may result in elevated serum levels, presumably by the release of enzyme from the damaged tissue. Thus, as with all laboratory tests, considered clinical judgment of the over-all picture remains essential. Despite this limitation, demonstration of elevated serum levels of certain enzymes (the two most useful at this time being SGOT and LDH) serves, in the proper clinical setting, as a highly valuable indicator of myocardial necrosis; failure to observe this on serial testing is an important negative finding. The SGOT activity rises promptly after myocardial infarction, permitting early confirmation. LDH, simpler to measure, affords the advantage of a more persisting abnormality (up to 10 to 14 days after infarction). In most comparative studies, LDH measurements appear to afford a greater over-all positive correlation with the ultimate diagnosis. The rapid development of this area of knowledge has already increased the diagnostic armamentarium of the clinician. Recent promise of methods to increase the specificity of the results augurs well for the future.