C4 (Complement Component )
Assess a key protein in your immune defences.
What’s tested
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Complement Component 4 (C4) is a protein that forms part of the complement system, a group of blood proteins that work together as part of your immune defences. The complement system helps the body fight infection, clear away damaged cells, and support the wider immune response. Measuring the level of C4 gives useful insight into how active this part of your immune system is.
What it can help assess
- Autoimmune conditions: C4 levels can fall when the complement system is being actively used up, as happens in conditions such as lupus (SLE). It is often used to help monitor disease activity.
- Angioedema: low C4 is a useful marker in investigating hereditary angioedema, a condition causing recurrent episodes of swelling.
- Kidney conditions: certain types of kidney inflammation (glomerulonephritis) can be associated with low C4 levels.
- Inflammation: as C4 is also an acute phase protein, levels can rise in response to infection or inflammation.
Why it matters
Measuring C4 helps give a picture of how active your complement system is. It is most often used alongside other tests, including Complement Component 3 (C3), to help investigate and monitor autoimmune conditions, recurrent swelling, and complement-related immune disorders.
No special fasting required. Drink plenty of water prior to your appointment for a smooth blood draw.
- Identification: Photo ID (Passport or Driving Licence) is only required if you need your report for official, employment, visa/travel, premarital, or Identity-Verified Certificate (IVS) purposes (not required for personal health & wellness testing).
This test provides an official, quality-assured laboratory report with reference ranges. These results are for clinical information and do not replace your GP or consultant. Always discuss abnormal results with your GP or specialist in the context of your medical history.
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