The judicious use of cardiac isoenzymes can clarify the diagnosis of myocardial infarction (MI), direct other cardiac studies, and prevent unnecessary tests. Elevated creatine kinase-MB (CK-MB) levels confirm that an MI with associated cell death has occurred. Lactic dehydrogenase levels are sometimes useful in evaluating patients who may have had an MI3 or 4 days earlier and whose serum CK levels have returned to normal.