Platform
Company
Complement
Review status
Currently under review
Pending specialist review and validation.
The C4 Complement test measures the amount of complement component 4 in your blood. C4 is a protein made mostly by the liver and is part of the complement system, a group of proteins that helps your immune system recognize and clear microbes and immune complexes. It is a key part of the classical complement pathway.
Doctors often order C4 together with other complement tests, such as C3 or a total complement activity test, to understand how active your immune system is and whether complement is being used up or produced at higher levels.
Changes in C4 can signal certain health conditions. Low C4 may occur when the classical complement pathway is activated by immune complexes, which can happen in some autoimmune diseases. Low C4 is also seen in some forms of hereditary angioedema and in a few rare inherited complement deficiencies. High C4 can occur during inflammation or with increased protein production.
Your clinician may order this test if you have symptoms that raise concern for autoimmune disease, immune complex disorders, recurrent swelling episodes, or certain infections. It can also be used to help monitor disease activity over time and to assess treatment response when complement abnormalities are part of your condition.
Your C4 level is interpreted alongside your symptoms, exam findings, and other laboratory tests. A low result can suggest that complement is being consumed by immune activation, or that there is an inherited or acquired problem affecting complement proteins. If C4 is low while other complement proteins are normal, that pattern can help narrow the cause. A higher result can be seen with inflammation, some medications, or hormonal states.
If your result is unexpected, your clinician may repeat the test and add related studies such as C3, total complement activity, or tests for C1 inhibitor if angioedema is suspected. Depending on your situation, additional evaluations for autoimmune disease, infection, or liver health may be recommended. Discuss any medications and supplements you take, since some can influence complement protein levels.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Prolonged time at room temperature or multiple freeze–thaw cycles can alter complement proteins. Proper collection, prompt processing, and appropriate storage help ensure reliable results.
Active infections and inflammatory conditions can change C4 production or consumption, which may raise or lower the measured level during illness or recovery.
Estrogen-containing therapies, pregnancy prevention pills, corticosteroids, and anabolic agents can increase complement proteins. Other drugs may indirectly lower levels by affecting the immune system.
Because C4 is made in the liver, significant liver disease or severe malnutrition can reduce production and lead to lower results independent of immune activity.
In people with suspected hereditary angioedema, C4 is often checked with C1 inhibitor tests. Levels may fluctuate with attacks and treatment, so timing of the blood draw matters.
Interpreting C4 together with C3 or total complement activity provides better insight into which complement pathway is affected and whether consumption is occurring.
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