Creatine Kinase-MB (CK-MB) is a protein found mostly in heart muscle which may be tested to determine if a patient has had a heart attack (elevated level) or if certain clot dissolving drugs are working. When the clot dissolves, CK-MB tends to rise and fall faster. By measuring CK-MB in blood several times, doctors can usually tell whether the drug has been effective. CK-MB may also be used in persons with high CK levels to determine if damage is to heart or other muscles.