Creatine kinase‐MB mass concentration versus creatine kinase‐B activity for the detection of acute myocardial infarction in patients with slightly elevated total creatine kinase activity in serum

OBJECTIVE: The diagnostic value of creatine kinase-MB mass concentration (CK-MB mass) was compared with that of creatine kinase-B (CK-B) activity in patients with suspected acute myocardial infarction (AMI) but with total serum CK activity only slightly above the reference range. DESIGN: One hundred consecutive blood samples with total CK activity between 120 and 360 U l-1 and CK-B activity > or = 9 U l-1 were analysed. Electrophoresis of CK isoenzymes was also performed. SETTING: Patients from all departments of the hospital were included. About half of the patients originated from the coronary care unit. SUBJECTS: The blood samples derived from 49 patients. Thirteen patients had at least one serum sample with total CK activity above 360 U l-1. These and another three patients were omitted from the study. RESULTS: Acute myocardial infarction had been diagnosed clinically (with CK and CK-B methods) in 12 of 33 patients. However, using the CK-MB mass concentration of the reference method, five of these 12 patients did not have myocardial infarction whereas nine patients with small infarctions were undetected. A good correlation was seen between the results from CK-MB mass concentration and CK isoenzyme electrophoresis, but there was a poor correlation between these methods and CK-B activity including the CK-B/CK ratio. A relatively high proportion (24%) of the selected patients had increased levels of macro CK. CONCLUSION: CK-B activity was inaccurate for the detection of probably myocardial infarction in patients with slightly elevated total CK activity. Increased levels of macro CK interfering with the CK-B assay was one explanation for this observation.