What is the C4 Complement Test?

The C4 Complement test measures the level of Complement Component 4, a protein that plays a central role in the body’s immune defence system. C4 is part of the classical complement pathway, which helps antibodies destroy pathogens and clear immune complexes from the bloodstream.

Complement proteins circulate in the blood and are activated during infection, inflammation or autoimmune processes. Abnormal C4 levels can indicate excessive immune activation, consumption due to autoimmune disease, or inherited complement deficiencies.

This test is commonly used alongside other complement markers, such as C3, to provide a clearer picture of immune system function.

Why do I need a test for C4 Complement?

You may need this test if autoimmune disease, chronic inflammation or immune dysfunction is suspected. Reduced C4 levels are often seen in autoimmune conditions where immune complexes consume complement proteins, while elevated levels may reflect acute inflammation.

C4 testing is particularly useful in the investigation and monitoring of systemic lupus erythematosus (SLE), vasculitis and hereditary angioedema. It also helps differentiate autoimmune disease activity from other inflammatory conditions.

Symptoms and scenarios where testing is recommended:

• Suspected autoimmune disease
• Systemic lupus erythematosus (SLE) monitoring
• Recurrent unexplained swelling (angioedema)
• Chronic inflammation
• Unexplained rashes or joint pain
• Low complement levels on previous testing
• Recurrent infections
• Immune system assessment

Benefits of the C4 Complement Test

• Supports diagnosis of autoimmune diseases
  Low C4 is commonly associated with immune complex disorders.
• Monitors disease activity in lupus
  Falling levels may indicate active disease or flare-ups.
• Assesses complement system function
  Helps identify inherited or acquired complement deficiencies.
• Differentiates immune from non-immune inflammation
  Adds specificity to inflammatory investigations.
• Useful in angioedema assessment
  Low C4 supports diagnosis of hereditary angioedema.
• Guides further immunological testing
  Results help determine need for additional complement studies.
• Supports long-term disease monitoring
  Serial measurements track immune activity over time.
• Simple but clinically informative test
  Requires only a standard blood sample.

Step-by-Step: How Testing Works

Choose your preferred testing method

You can attend one of our UK partner clinics or arrange a home or hotel visit with a mobile phlebotomist.

Option 1 – In-Clinic Blood Draw

1. Select your nearest UK clinic and book an appointment.
2. A clinician or phlebotomist collects a venous blood sample.
3. The sample is analysed for C4 complement levels.
4. Results are delivered securely online.

Option 2 – Home or Hotel Phlebotomy (+£60)

1. Book a UK-wide home or hotel visit at a convenient time.
2. A trained phlebotomist collects your blood sample on-site.
3. The sample is packaged and transported to the laboratory.
4. Results are uploaded digitally without the need to travel.

Access your report

You can download and share your results with your GP, immunologist or rheumatologist.

Your Results Explained

Low Levels

Low C4 levels suggest increased consumption or deficiency of complement proteins.
This is commonly seen in autoimmune diseases such as systemic lupus erythematosus or in hereditary angioedema.
Typical UK reference range: approximately 0.14–0.54 g/L (ranges vary by laboratory).

Normal Levels

Normal C4 levels indicate balanced complement activity.
This suggests no significant immune complex consumption at the time of testing.

High Levels

Elevated C4 levels may reflect acute inflammation or infection.
High levels are generally non-specific and should be interpreted alongside other inflammatory markers.