Creatine kinase (CK), also known as creatine phosphokinase (CPK), is an enzyme found mainly in skeletal muscle, the heart and the brain.

CK is a sensitive marker of muscle damage. Significant elevation of CK is often observed in rhabdomyolysis, a condition characterised by the destruction of muscle cells, which can lead to acute renal failure if not treated promptly.

Significant physical trauma or intense exercise can cause a transient elevation in CK, indicating muscle damage.

Historically, CK, and more specifically the CK-MB isoenzyme, was used to diagnose myocardial infarction. Although cardiac troponins have now replaced CK-MB as the main marker of cardiac damage, CK can still be measured in certain situations to assess myocardial damage.

Statins, drugs used to lower cholesterol, can cause statin-induced myopathy, a condition characterised by muscle pain and elevated CK. CK measurement is therefore used to monitor these side effects and adjust treatment if necessary.

The C4 complement assay is a laboratory test that measures the level of C4 protein in the blood, an important component of the complement system, which is part of the immune system. As with C3, the C4 assay is used to diagnose and monitor various autoimmune and inflammatory diseases and other pathological conditions.
Normal levels of C4 vary between 10 and 40 mg/dL.

Decreased C4 levels suggest increased consumption due to complement activation, typically in the context of autoimmune or chronic inflammatory diseases. Elevated levels may indicate an acute inflammatory response.

Like C3, C4 can be used to monitor transplant rejection, particularly in the early stages when complement may be activated.