What is the C3 Complement Test?

The C3 Complement test measures the level of Complement Component 3, one of the most important proteins in the complement system. C3 plays a central role in all major complement pathways (classical, alternative and lectin), helping the immune system identify, attack and clear pathogens and immune complexes.

Because C3 is involved in multiple immune pathways, abnormal levels can reflect increased immune activation, consumption during autoimmune disease, or inherited complement deficiency. It is one of the most commonly measured complement proteins in clinical practice.

C3 is often tested together with C4 to help distinguish between different types of immune and inflammatory processes.

Why do I need a test for C3 Complement?

You may need this test if autoimmune disease, chronic inflammation or immune dysfunction is suspected. Reduced C3 levels often indicate complement consumption due to immune complex formation, while elevated levels may reflect acute inflammation or infection.

C3 testing is particularly useful in the diagnosis and monitoring of systemic lupus erythematosus (SLE), glomerulonephritis and other immune-mediated conditions. Changes in C3 over time can provide valuable insight into disease activity and response to treatment.

Symptoms and scenarios where testing is recommended:

• Suspected autoimmune disease
• Monitoring systemic lupus erythematosus (SLE)
• Chronic or unexplained inflammation
• Recurrent infections
• Kidney disease linked to immune causes
• Low complement levels on previous tests
• Immune system assessment
• Vasculitis or immune complex disorders

Benefits of the C3 Complement Test

·      Core marker of immune system activity
Reflects activation or consumption of the complement system.

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

·      Monitors disease activity over time
Useful for tracking flares and treatment response.

·      Assesses complement pathway integrity
Helps identify inherited or acquired complement deficiencies.

·      Useful in kidney-related immune disease
Often abnormal in immune-mediated glomerulonephritis.

·      Differentiates immune from non-immune inflammation
Adds specificity to inflammatory investigations.

·      Guides further immunological testing
Helps determine need for extended complement analysis.

·      Simple yet 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 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 levels suggest 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 range: approximately 0.90–1.80 g/L (ranges vary by laboratory).

Normal Levels

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

High Levels

Elevated C3 levels may reflect acute inflammation, infection or tissue injury.
High levels are generally non-specific and should be interpreted alongside other inflammatory markers.