Complement C3 blood test

Complement C3 is one of the nine main proteins that make up the complement system — a part of innate immunity that helps the body recognise and destroy bacteria and other pathogens. C3 is the most abundant complement protein in the blood and the central link in this system: it is activated by three different pathways (the classical, the alternative and the lectin pathway) and, once broken into fragments, helps tag foreign cells for destruction and clear immune complexes from the blood. Because the complement system is closely tied to the course of autoimmune processes, C3 (often together with C4) is tested when assessing conditions such as systemic lupus erythematosus or rheumatoid arthritis — both to diagnose them and to monitor disease activity over time. Per MedlinePlus, the amount of C3 in the blood can change both because of the inflammatory process itself and because complement proteins are consumed during immune reactions, so the result is always interpreted by a doctor together with other clinical data.

• Where an autoimmune disease such as systemic lupus erythematosus or rheumatoid arthritis is suspected, or is being monitored
• To assess disease activity and how well treatment is working in people already diagnosed with lupus
• Where hereditary angioedema or recurrent bacterial infections are suspected and the complement system may be impaired
• In liver disease or kidney damage, where the production or the consumption of complement proteins may be disturbed
• Together with other inflammation and autoimmune markers (CRP, ANA, rheumatoid factor) where the picture of symptoms is more complicated

Per MedlinePlus (Complement Blood Test), the page gives no specific reference interval with units for complement C3 — the reference limits depend on the laboratory method, so the exact interval has to be looked up on your own laboratory result sheet.

A raised complement C3 is linked by MedlinePlus to chronic inflammation, some blood cancers (leukaemia, lymphoma) and ulcerative colitis. In people with lupus, a rising C3 level during treatment can indicate that the treatment is working effectively, because less of the complement system is being consumed in inflammatory processes.

• Chronic inflammation in the body
• Some blood cancers — leukaemia, lymphoma
• Ulcerative colitis
• A sign that lupus treatment is working — complement levels rising while disease activity falls

Alongside a raised C3, these are the things usually assessed:

• Inflammation markers (CRP, ESR) are assessed alongside, to pin down the cause of the inflammation
• In lupus or another autoimmune disease — the result is compared with earlier tests, to follow disease activity
• Whether there are bowel symptoms that could point to ulcerative colitis
• When to repeat the test to assess the change over time

A low C3: what it can mean

A low complement C3 is, per MedlinePlus, most often linked to an active autoimmune disease — lupus or rheumatoid arthritis — and also to liver cirrhosis, kidney disease, hereditary angioedema, poor nutrition or recurrent bacterial infections. A low C3 can mean that complement proteins are being heavily consumed as an immune reaction takes place.

• Active systemic lupus erythematosus or rheumatoid arthritis
• Liver cirrhosis
• Kidney disease
• Hereditary angioedema
• Poor nutrition
• Recurrent bacterial infections

Alongside a low C3, the questions are different:

• Liver function markers and kidney markers, to pin down the possible cause
• Autoimmune disease markers (ANA, rheumatoid factor) together with complement C4
• The history of infections and how often they occur
• When to repeat the test to follow disease activity