Biochemical Markers Cardiac Enzymes

Biochemical markers of myocardial cell death are important for confirming the diagnosis of MI. The diagnosis of MI is confirmed when the following conditions are met in a clinical setting consistent with myocardial ischemia: "Detection of a rise and/or fall of cardiac biomarkers (troponin preferred) with at least one value above the 99th percentile of the upper reference limit together with evidence of myocardial ischemia as recognized by at least one of the following: (a) symptoms of ischemia; (b) ECG changes of new ischemia or development of pathological Q waves; or (c) imaging evidence of new loss of viable myocardium or new regional wall motion abnormality. "10 Typically, a blood sample is obtained once in the emergency department, then 6 to 9 hours later, and in patients at a high suspicion of MI but in whom previous measurements did not reveal elevations in biomarkers, 12 to 24 hours after. A single measurement of a biochemical marker is not adequate to exclude a diagnosis of MI, as up to 15% of values which were initially below the level of detection (a "negative" test) rise to the level of detection (a "positive" test) in subsequent hours. While troponins and CK-MB appear in the blood within 6 hours of infarction, troponins stay elevated for up to 10 days while CK-MB returns to normal values within 48 hours. Hence, traditionally, CK-MB was used to detect reinfarction. However, more recent data have suggested that troponins provide similar information to CK-MB in such a situation which has lead to the use of troponins in this setting as well. Current guidelines suggest that, in patients in whom a recurrent MI is suspected, a cardiac biomarker should be immediately measured, followed by a second measurement three to six hours later. A recurrent MI is diagnosed when there is an increase of at least 20% in the second measurement of the biomarker, if this value exceeds the 99th percentile of the upper reference limit.10

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