Creatine kinase Isozyme (Activity Test) is an in vitro diagnostic quantitative test kit. Creatine kinase (CK) is an enzyme, found primarily in muscle and brain tissue. CK-MB is one of the most important myocardial markers (in spite of not being altogether cardiac-specific), with well-established roles in confirming acute myocardial infarction (AMI) and in monitoring reperfusion during thrombolytic therapy following AMI.