C4 - Clinical: Complement C4, Serum

Test Catalog

Test Name

Test ID: C4    
Complement C4, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Investigating an undetectable total complement (CH50)


Confirming hereditary angioedema (with low C1 inhibitor)


Assessing disease activity in systemic lupus erythematosus, proliferative glomerulonephritis, rheumatoid arthritis, and autoimmune hemolytic anemia

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

The complement system is an integral part of the immune defenses. It can be activated via immune complexes (classic pathway) or by bacterial polysaccharides (alternative pathway). The classic complement pathway consists of recognition, (C1q, C1r, C1s), activation (C2, C3, C4), and attack (C5, C6, C7, C8, C9) mechanisms with respect to their role in antibody-mediated cytolysis. C4 is one of the activation proteins of the classic pathway.


In the absence of C4, immune complexes will not be cleared by C3 activation peptides, but bacterial infections can still be defended via the alternative pathway.


C4 may be decreased in systemic lupus erythematosus, early glomerulonephritis, immune complex disease, cryoglobulinemia, hereditary angioedema, and congenital C4 deficiency.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

14-40 mg/dL

Interpretation Provides information to assist in interpretation of the test results

C4 levels will be decreased in acquired autoimmune disorders, in active phase of lupus erythematosus, and in rheumatoid arthritis


An undetectable C4 level (with normal C3) suggests a congenital C4 deficiency


Levels will be increased in patients with autoimmune hemolytic anemia

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

The results are dependent on appropriate specimen transport.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Ross SC, Densen P: Complement deficiency states and infection: epidemiology, pathogenesis, and consequences of neisserial and other infections in an immune deficiency. Medicine 1984;63:243-273

2. Frank MM: Complement in the pathophysiology of human disease. N Engl J Med 1987;316:1525-1530

3. Tiffany TO: Fluorometry, nephelometry, and turbidimetry. In Textbook of Clinical Chemistry. Edited by NW Tietz. Philadelphia, WB Saunders Company, 1986, pp 79-97