Test Catalog

Test ID: C3    
Complement C3, Serum

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

Assessing disease activity in systemic lupus erythematosus (SLE)


Investigating an undetectable total complement (CH50) level

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 body's immune defenses. The primary complement pathway consists of recognition (Clq, Clr, Cls), activation (C4, C2, C3), and attack (C5, C6, C7, C8, C9) mechanisms with respect to their role in antibody-mediated cytolysis.


The complement system can be activated via immune complexes, and the alternative pathway (properdin pathway), which is activated primarily by foreign bodies such as microorganisms.


C3 activation involves cleavage by C3 convertase into C3a and C3b. When immune complexes are not involved, the alternate method of complement activation initiates the reactant sequence at C3, bypassing C1, C4, and C2.


Severe recurrent bacterial infections occur in patients with homozygous C3 deficiency and in those patients with low levels of C3 secondary to the absence of C3b activator.


Decreased C3 may be associated with acute glomerulonephritis, membranoproliferative glomerulonephritis, immune complex disease, active systemic lupus erythematosus, septic shock, and end-stage liver disease.

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.

75-175 mg/dL

Interpretation Provides information to assist in interpretation of the test results

A decrease in C3 levels to the abnormal range is consistent with disease activation in systemic lupus erythematosus (SLE).

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 and storage.

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