A 45-year-old man experiences crushing substernal chest pain after arriving at work one morning. Over the next 4 hours, the pain persists and begins to radiate to his left arm. He becomes diaphoretic and short of breath but waits until the end of his 8-hour shift to go to the hospital. An elevated serum value of which of the following laboratory tests would be most useful for the diagnosis of this patient on admission to the hospital?

Of the enzymes listed, CK-MB is the most specific for myocardial injury from the acute myocardial infarction (MI) described in this patient. The levels of this enzyme begin to increase within 2 to 4 hours of ischemic myocardial injury. ALT elevation is more specific for liver injury. AST is found in various tissues; elevated levels are not specific for myocardial injury. The elevation of lactate dehydrogenase (LDH)-1 compared with LDH-2 suggests myocardial injury, but LDH activity peaks 3 days after an MI. C-reactive protein is elevated with inflammatory processes, but is nonspecific; it has been used as a predictor of acute coronary syndromes. Lipase is a marker for pancreatitis.