Creatine Kinase (CK)

CK is an enzyme. An enzyme is a protein that speeds up certain chemical reactions in your body. CK helps make energy. Most of the CK in your body is in your skeletal muscles. These are the muscles you use to move. You also have CK in your heart muscle and small amounts in your brain. CK is made up of two subunits, which combine to form three different forms: one mainly in the brain, one mainly in skeletal muscle, and one mainly in the heart.

Normally, a small amount of CK gets into your blood from the usual wear and tear on your muscles. But if your muscles, heart, or brain tissues are damaged, larger amounts of CK leak out of your cells into your bloodstream.

A CK test is mainly used to help diagnose and monitor injuries and diseases that damage skeletal muscles and cause high levels of CK in your blood. But it may also be used for conditions that damage the heart muscle and the brain.

If your CK level is higher than normal, it usually means you have some type of muscle damage. CK levels may not reach their highest until up to two days after certain injuries. So, you may have more than one CK test to see if your levels go up or down. CK levels that stay high or increase may mean that muscle damage is continuing to happen.

Common reasons for raised CK include heavy exercise, muscle injuries, falls, surgery, an underactive thyroid, and certain medicines such as statins (used to lower cholesterol) and some other prescription drugs. Both tough workouts and large amounts of caffeine or supplements can also temporarily raise CK.

Very high CK — many times the upper limit of normal — can be a sign of rhabdomyolysis, a condition where muscle breaks down so quickly that it can damage the kidneys. Rhabdomyolysis is a medical emergency and needs urgent care.

But a CK test result can't show where the damage is or what's causing it. Your provider will consider your symptoms and medical history to understand what your test results mean.

If the source of high or increasing CK levels isn't clear, you may need a more specific type of CK test to find out if the CK is coming from your muscles, heart, or brain. This more specific test is called a CK isoenzymes test.