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Test ID: C3 Complement C3, Serum

Reporting Name

Complement C3, S

Useful For

Assessing disease activity in systemic lupus erythematosus (SLE)


Investigating an undetectable total complement (CH50) level

Clinical Information

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.


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

Report Available

Same day/1 to 2 days

Day(s) Performed

Monday through Friday

Clinical Reference

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

Method Name


Specimen Type


Specimen Required


Preferred: Serum gel

Acceptable: Red top

Specimen Volume: 1 mL

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 7 days
  Frozen  28 days
  Ambient  72 hours

Reference Values

75-175 mg/dL

Test Classification

This test has been cleared, approved, or is exempt by the US Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
C3 Complement C3, S 4485-9


Result ID Test Result Name Result LOINC Value
C3 Complement C3, S 4485-9


If not ordering electronically, complete, print, and send a Renal Diagnostics Test Request (T830) with the specimen.

Mayo Clinic Laboratories | Neurology Catalog Additional Information:

mml-Cerebrovascular, mml-Headache, mml-Demyelinating-Diseases, mml-Neuroimmunology, mml-Neuromuscular, mml-Pediatric, mml-Spinal-Cord