What is creatine kinase?

Creatine kinase, or CK, is an enzyme found primarily in the brain, heart, skeletal muscles, and other tissues. CK, also called phosphocreatine kinase or CPK, helps cells produce a biochemical reaction that produces high-energy molecules that cells use to perform normal functions. When creatine kinase combines with adenosine triphosphate (ATP) it produces phosphocreatine and adenosine triphosphate (ATP). Muscles use these energy molecules to contract muscle fibers. This reaction can also work in reverse, so that phosphocreatine and ADP can create ATP.

There are three types of creatine kinase, or isoenzymes, produced by the body. CK-MM is produced by skeletal muscle, CK-MB is produced by the heart, and CK-BB is produced by the brain. Under certain circumstances, CK-MM can leak from cells and enter the bloodstream. This happens when the muscle is damaged, either by muscle injury, exercise, or as a result of a neuromuscular disorder.

Doctors can test for the presence of creatine kinase in the blood to help diagnose and evaluate disease in which muscle degeneration may occur. A blood serum sample is obtained and CK levels are measured in enzyme units per liter. Levels generally vary based on gender, activity levels, and other factors, but a normal range is 22 to 198 units per liter.

High levels of creatine kinase may indicate a neuromuscular disease such as muscular dystrophy. Elevated CK may also indicate heart attack, acute kidney failure, polymyositis, dermatomyositis, hypothyroidism, hypothermia, recent surgery, or a recent bout of vigorous exercise. In the case of neuromuscular disease, high levels of CK can diagnose the disease before the appearance of other symptoms.

In cases of rhabdomyolosis, the muscles break down very quickly, releasing cellular contents into the bloodstream. When this happens, creatine kinase levels can become very high, indicating a dangerous condition where types of proteins called myoglobins can enter the kidneys. Renal failure may occur. Rhabdomyolosis is not a condition in itself, but rather a symptom of other conditions in the body. It most commonly occurs after a serious injury or as a result of medication and requires immediate medical attention.

People who take statins to control cholesterol may need to have their CK levels checked regularly. A side effect of statins is muscle pain and weakness which, in rare cases, can lead to rhabdomyolosis and risk of kidney failure. Close monitoring of creatine kinase levels can alert the doctor to early signs of muscle breakdown, so kidney damage can be prevented.

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