Complement 4 (C4)

Complement component C4 is a blood protein in the classical and lectin complement pathways, produced mainly by the liver. It’s measured to detect immune activation or deficiency. Low C4 suggests complement consumption or hereditary deficiency seen in autoimmune conditions (like lupus), immune-complex diseases, and hereditary angioedema; elevated C4 is uncommon. Results should be interpreted with symptoms and other tests by a clinician.

High complement C4a indicates activation of the classical complement pathway and ongoing immune/inflammatory activity. It can occur with infections, autoimmune disorders (such as lupus or rheumatoid disease), immune-complex conditions, or severe allergic reactions. Isolated elevation requires clinical correlation and further testing (C3, C4, C1 inhibitor, autoantibodies, infection workup). Discuss results with your clinician to identify and treat the cause.

A low C4 level—below the laboratory reference range (often <15–20 mg/dL)—suggests complement consumption seen in systemic lupus erythematosus; levels under about 10–15 mg/dL are commonly associated with active disease. Low C4 supports, but does not by itself diagnose lupus; results must be interpreted alongside clinical findings and other tests (e.g., ANA, anti‑dsDNA).

When C4 is low, it indicates activation or deficiency of the classical and lectin complement pathways. Clinically this can cause recurrent bacterial infections, impaired clearance of immune complexes with higher risk of autoimmune disease (such as systemic lupus erythematosus), and—when linked to C1 inhibitor deficiency—recurrent angioedema. Low C4 with normal C3 points to classical-pathway consumption; further immunologic testing is advised.

Testing for lupus begins with a detailed history and physical exam, followed by blood and urine tests. Key labs include ANA screening, anti-dsDNA and anti‑Smith antibodies, complement (C3/C4) levels, CBC, ESR/CRP, and urinalysis for protein or blood. Imaging and organ-specific tests assess damage; a kidney biopsy may be needed if lupus nephritis is suspected. See a rheumatologist for diagnosis.

C4, a complement protein, plays a role in lupus. Genetic C4 deficiency or chronically low C4 impairs clearance of immune complexes and apoptotic cells, promoting autoimmunity and increasing risk of systemic lupus erythematosus. Low C4 (often with low C3) also correlates with disease activity and is used clinically to help diagnose and monitor lupus flares.