In which condition would creatine kinase (CK) be normal?

• A. Acute myocardial infarct
• B. Hepatitis
• C. Progressive muscular dystrophy
• D. Intramuscular injection

Answer: (B)

Creatine kinase is an enzyme abundant in muscle tissue (skeletal and cardiac) and in the brain, and it leaks into the blood when muscle cells are damaged. Because of that, CK levels rise with conditions that injure muscle or heart muscle, such as a heart attack (where CK-MB from the heart increases) or chronic muscle diseases like muscular dystrophy, where ongoing muscle breakdown releases more CK. An intramuscular injection can cause temporary muscle injury and a transient CK rise as well. Hepatitis, however, is injury to liver cells, not muscle tissue, so CK is not a marker for liver injury and typically remains normal. Liver injury is usually reflected by elevated liver enzymes such as ALT and AST. Therefore, CK would be normal in hepatitis.