If myopathy is present, which laboratory value is typically markedly elevated?

• A. Creatinine
• B. Creatine kinase
• C. ALT
• D. Bilirubin

Answer: (B)

When muscle tissue is damaged, enzymes spill into the bloodstream, and the most prominent marker of muscle injury is creatine kinase. CK is abundant in skeletal muscle, so injury or disease of muscle causes a marked rise in the blood, often far exceeding normal ranges in myopathies and especially in rhabdomyolysis. The different isoforms explain why CK, particularly CK-MM from skeletal muscle, rises with muscle damage, whereas the other markers are not as specific to muscle injury.

Creatinine reflects kidney function and can be influenced by muscle mass, but it isn’t a direct indicator of muscle injury. ALT is mainly a liver enzyme (though mild elevations can occur with nonhepatic conditions), and bilirubin relates to liver or biliary processing and red blood cell turnover.