C3 Complement, Functional, Serum

CPT CODE:

USEFUL FOR:

Diagnosis of C3 deficiency
Investigation of a patient with undetectable total complement (CH[50]) level

SPECIMEN REQUIRED:

Draw blood in a plain, red-top tube(s) or a serum gel tube(s) froma fasting patient. Spin down, separate from clot, and send 1 mL of serum frozen in plastic vial.

TRANSPORT TEMPERATURE:

Frozen\Refrig NO\Ambient NO

CLINICAL INFORMATION:

Complement proteins are components of the innate immune system. There are 3 pathways to complement activation: 1) theclassic pathway, 2) the alternative (or properdin) pathway, and 3)the lectin activation (mannan-binding protein [MBP]) pathway. Theclassic pathway of the complement system is composed of a seriesof proteins that are activated in response to the presence of immunecomplexes. The activation process results in the generation ofpeptides that are chemotactic for neutrophils and that bind to immunecomplexes and complement receptors. The end result of thecomplement activation cascade is the formation of the lytic membrane attack complex (MAC). 
The absence of early components (C1-C4) of the complementcascade results in the inability of immune complexes to activatethe cascade. Patients with deficiencies of the early complementproteins are unable to clear immune complexes or to generatelytic activity. These patients have increased susceptibility to infections with encapsulated microorganisms. They may also havesymptoms that suggest autoimmune disease and complementdeficiency may be an etiologic factor in the development of autoimmune disease.
C3 is at the entry point for all 3 activation pathways to activate theMAC. C3 deficiency may result in pneumococcal and neisserialinfections as well as autoimmune diseases such as glomerulonephritis.
Complement levels can be detected by antigen assays that quantitate the amount of the protein (#8174 "Complement C3,Serum"). For most of the complement proteins, a small number ofcases have been described in which the protein is present but isnon functional. These rare cases require a functional assay todetect the deficiency.

CLINICAL INTERPRETATION:

Low levels of complement may be due to inherited deficiencies, acquired deficiencies, or due to complement consumption (e.g., as a consequence of infectious or autoimmune processes).
Absent C3 levels in the presence of other normal complementvalues are consistent with a C3 deficiency.

REFERENCE VALUES:

21-50 unit/mL