Complement C4 blood test in Chiswick

What the test measures

C4 is one of the proteins in the classical pathway of the complement system, the route usually triggered when antibodies bind to a target. Activating this pathway uses up C4, so its level in the blood gives information about how active that part of the immune system is. Like C3, it is mostly made in the liver.

C4 is commonly measured with C3. Low C4 with a normal C3 can point to activity in the classical pathway, and C4 is also used as a first test when hereditary angioedema is suspected, a rare condition that causes repeated swelling of the skin, gut or airway. Some people naturally have lower C4 because of inherited differences in the genes that make it. The price includes the blood sample taken at our Chiswick clinic, analysis at the Randox accredited laboratory, a review of your result with an Omnia practitioner and a GP feedback report on request.

Who it is for

• You have lupus or another autoimmune condition and your clinician monitors complement levels.
• You have episodes of swelling of the face, lips, hands or gut without hives and want to look at possible causes.
• You have a family history of hereditary angioedema.
• You have kidney inflammation and complement testing has been suggested.
• You have recurring infections and are exploring your immune function with a practitioner.

Understanding your result

Your report shows your C4 with the laboratory reference range. A low C4 can suggest the classical pathway is being used up, which is seen in active lupus, some vasculitis and kidney conditions, cryoglobulinaemia and hereditary angioedema. It can also be naturally low in some healthy people. A raised C4 is usually a sign of inflammation.

If hereditary angioedema is suspected, further tests are needed and are usually arranged by an immunology specialist. Any swelling of the tongue or throat, or difficulty breathing, needs a 999 call. An out-of-range result is something your practitioner will explain, not a diagnosis.

Preparing for your test

No fasting is needed for C4. It is helpful to test when you are well rather than during an infection. Let us know about any medicines you take, including steroids, hormones or immune treatments, and any family history of swelling disorders.

C4 is included with C3 and other immune markers in Advanced GP3 and in Omnia Signature. Our main page lists every blood test and price in Chiswick.

Questions

How much is a complement C4 test in Chiswick?

The complement component 4 (C4) test costs £79. The price includes your sample at our Chiswick clinic, laboratory analysis, a practitioner review and a GP feedback report if requested.

What does a low C4 mean?

It can mean the classical complement pathway is active, as in some autoimmune conditions, or point towards hereditary angioedema. Some people simply have naturally low C4.

Can C4 diagnose hereditary angioedema?

Not on its own. A low C4 is a common first clue, but the diagnosis needs further specialist tests of C1 inhibitor level and function.

How is C4 different from C3?

C4 belongs to the classical pathway, while C3 is shared by all pathways. Comparing the two helps show which part of the system is involved.