What is the C3 + C4 Complement Test?

The C3 + C4 Complement test measures the blood levels of Complement Component 3 (C3) and Complement Component 4 (C4), two essential proteins involved in the body’s immune defence system. Together, they play a central role in identifying and clearing infections, immune complexes and damaged cells.

C3 is involved in all major complement pathways, while C4 is primarily part of the classical pathway, which is activated by antibodies. Measuring both markers together provides a more complete picture of complement system activity than testing either one alone.

This combined test is widely used in immunology and rheumatology to evaluate immune-mediated conditions and monitor disease activity over time.

Why do I need a test for C3 + C4 Complement?

You may need this test if autoimmune disease, chronic inflammation or immune dysfunction is suspected. Changes in C3 and C4 levels help identify whether the immune system is overactive, consuming complement proteins, or functioning normally.

Testing both complements together is particularly important in conditions such as systemic lupus erythematosus (SLE), vasculitis and immune-mediated kidney disease. Patterns of low C3, low C4 or both can help clinicians understand the underlying immune mechanism and guide further investigation.

Symptoms and scenarios where testing is recommended:

• Suspected autoimmune disease
• Monitoring systemic lupus erythematosus (SLE)
• Chronic or unexplained inflammation
• Recurrent infections
• Immune-related kidney disease
• Unexplained rashes or joint pain
• Low complement levels on previous tests
• Assessment of immune system activity

Benefits of the C3 + C4 Complement Test

·      Provides a comprehensive view of complement activity
Measures two key proteins involved in immune defence.

·      Supports diagnosis of autoimmune diseases
Low levels are common in immune complex disorders.

·      Monitors disease activity and flares
Changes over time reflect autoimmune activity.

·      Helps differentiate immune pathways involved
Patterns of C3 and C4 changes aid interpretation.

·      Useful in lupus and vasculitis monitoring
Widely used in rheumatology practice.

·      Assists in immune-related kidney disease assessment
Often abnormal in glomerulonephritis.

·      Guides further immunological testing
Indicates need for extended complement studies.

·      Simple but highly informative blood 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 C3 and 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 C3 and/or C4 levels indicate increased complement consumption or deficiency.
This is commonly seen in autoimmune diseases such as systemic lupus erythematosus or immune-mediated kidney disease.
Typical UK reference ranges:

• C3: ~0.90–1.80 g/L
• C4: ~0.14–0.54 g/L

Normal Levels

Normal C3 and C4 levels indicate balanced complement activity.
This suggests no significant immune complex consumption at the time of testing.

High Levels

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