Creatine kinase:aspartate aminotransferase activity ratio as an indicator of the source of an increased creatine kinase activity.

Although measurements of creatine kinase isoenzyme 2 (CK-MB) are often used to diagnose acute myocardial infarction, their sensitivity and specificity are less than 100%. Because skeletal muscle contains more CK and less aspartate aminotransferase (AST) than cardiac muscle, the CK/AST ratio might provide a useful adjunct in evaluating the source of a supranormal value for CK. I established the following decision levels in a retrospective study of 342 patients: ratios less than 14 (if total CK was 300-1200 U/L), less than 20 (CK 1201-2000 U/L), or less than 25 (CK greater than 2000 U/L) suggested myocardial infarction, with a sensitivity of 95% and a specificity of 65%. In a validation study with 277 additional patients, liver disease and alcohol abuse caused erroneous results, leading to exclusion of 22% of these patients. In the remaining cases, sensitivity was 94%, specificity 90%. The CK/AST ratios changed little with time, suggesting that a single value would be adequate for evaluating patients with increased CK.