CK (creatine kinase) is an enzyme (protein) found in several tissues, including muscles and the heart. Depending on the tissue, different forms of CK are present: CK-MM is primarily present in skeletal muscles, CK-MB represents 30% of CK from the heart, while CK-BB comes from the brain and smooth muscle, such as the intestinal walls. Atypical forms of CK (macro CK1 and macro CK2) can also be present. CK electrophoresis is most useful when muscular or cardiac disease does not seem to be responsible for the increased level of total CK.
A percentage of CK-MM greater than 95% indicates that the increase in total CK was muscular in origin. A percentage of CK-MB greater than 5% indicates that the heart was likely involved. An increase in the percentage of CK-BB is very rare and can reflect brain damage or certain types of cancer. The presence of macro CK type 1 (macro CK1), a large-sized CK that stays longer in the blood, is most often associated with the presence of autoimmune diseases. Macro-CK2 is sometimes found in certain cancers.