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Comparison of the effectiveness of four clinical chemical assays in classifying patients with chest pain.

We compared the usefulness of four serum assays for classifying patients originally suspected of having an acute myocardial infarction. One of these is the long-used measurement of total creatine kinase (CK) activity. The other three are relatively new immunoassays: myoglobin by RIA, CK-BB by RIA, and CK-MB by immunoinhibition. When we evaluated test effectiveness with use of conventionally derived reference ranges, the results were misleading. However, by using receiver operating characteristic curves, we were able to effectively compare the four tests at all possible decision levels, rather than at only one. Multiple closely sequential serum specimens were obtained during the first four days after the onset of chest pain. Total CK, CK-MB, and CK-BB all behaved similarly, reaching peak diagnostic effectiveness at 18-20 h, when all three correctly classified 95% of the infarct patients, with a zero false-positive rate. However, total CK was more useful in identifying infarcts later in their courses than were the two CK isoenzyme tests. Myoglobin assay was most effective earlier in the course, at about 7 to 8 h. Our results indicate (a) that the tests for myoglobin and for CK or its isoenzymes are complementary and (b) that of the three CK tests, measurement of total CK activity provides the most information over the broadest segment of a patient's course.