CK (creatine kinase) — also called CPK — is a blood test that measures an enzyme released by muscle cells when they are damaged or under stress. Here is what your result means.
Your lab may print slightly different numbers — always use the reference interval on your result. CK levels vary with exercise, muscle mass, and ethnicity.
Creatine kinase (CK) is an enzyme found mainly in muscle cells — skeletal muscle, heart muscle, and the brain. Its job is to help muscle cells regenerate energy for contraction.
When muscle cells are damaged — by exercise, injury, inflammation, or disease — CK leaks into the bloodstream. A blood test can then detect it.
| Isoenzyme | Where it comes from | Why it matters |
|---|---|---|
| CK-MM | Skeletal muscle | Makes up ~95% of total CK in healthy people. Rises after exercise, muscle injury, or muscle disease. |
| CK-MB | Heart muscle | Raised in heart attack. Largely replaced by troponin as the preferred cardiac marker, but still used in some contexts (e.g. suspected reinfarction). |
| CK-BB | Brain, smooth muscle | Rarely measured clinically. May be elevated in brain injury. |
A standard CK blood test measures total CK. If your GP suspects a heart attack, they will order troponin and CK-MB separately.
| Level (approx.) | Likely meaning | What usually happens next |
|---|---|---|
| <250 U/L (M) / <150 U/L (F) | Normal | No action needed |
| 250–500 U/L | Mildly elevated — often exercise | GP may repeat after 48 hrs rest; review medications |
| 500–1,000 U/L | Moderately elevated | Investigate cause — consider statin myopathy, thyroid function |
| 1,000–10,000 U/L | Significantly elevated | GP review warranted — consider myositis, statin toxicity |
| >10,000 U/L | Rhabdomyolysis range | Hospital assessment is often required — risk of acute kidney injury |
These are general guides only. Your GP interprets your CK in the context of your symptoms, medications, activity level, and other test results. A single number alone is rarely enough to make a diagnosis.
Statins (e.g. atorvastatin, rosuvastatin, simvastatin) can rarely cause muscle inflammation. Most people on statins have no muscle symptoms and normal CK. New muscle pain, tenderness, or weakness while on a statin is something GPs commonly investigate with a CK test. A CK result above 10× the upper limit of normal is generally considered a significant finding in this context. Any changes to statin therapy are a decision for your GP based on your full clinical picture.
Males generally have greater skeletal muscle mass, which means there is more CK in their bodies at baseline. This is why male reference ranges are higher. CK also tends to be higher in people of African or Indigenous ethnicity, and in people with high muscle mass from exercise or physical work.
No. Fasting is not required for a CK test. However, avoiding strenuous exercise for 48–72 hours before the test gives a more accurate baseline result. A CK measured the morning after an intense workout will be significantly higher than your true resting level.
Rhabdomyolysis is severe, rapid breakdown of skeletal muscle that releases large amounts of CK, myoglobin, and other substances into the blood. CK is usually above 10,000 U/L — sometimes in the hundreds of thousands.
The main danger is that myoglobin can block and damage the kidneys. Symptoms include severe muscle pain, weakness, and dark brown or red urine (like cola). Causes include crush injury, extreme exercise (e.g. first marathon, military training), drug toxicity, severe infections, seizures, and heat stroke.
Severe muscle pain combined with dark brown or red urine is a presentation GPs and emergency departments treat urgently, as there is a risk of kidney injury.
A low CK is rarely clinically significant. It can occur in small-framed individuals, during pregnancy, or in people with very low muscle mass. It is not generally a cause for concern.