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Unit Code 81090:
C3 Complement, Functional, Serum

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Useful For

Diagnosis of C3 deficiency

 

Investigation of a patient with undetectable total complement

(CH[50]) level

Clinical Information

Complement proteins are components of the innate immune

system. There are 3 pathways to complement activation: 1) the

classic pathway, 2) the alternative (or properdin) pathway, and 3)

the lectin activation (mannan-binding protein [MBP]) pathway. The

classic pathway of the complement system is composed of a series

of proteins that are activated in response to the presence of immune

complexes. The activation process results in the generation of

peptides that are chemotactic for neutrophils and that bind to immune

complexes and complement receptors. The end result of the

complement activation cascade is the formation of the lytic

membrane attack complex (MAC). 

 

The absence of early components (C1-C4) of the complement

cascade results in the inability of immune complexes to activate

the cascade. Patients with deficiencies of the early complement

proteins are unable to clear immune complexes or to generate

lytic activity. These patients have increased susceptibility to

infections with encapsulated microorganisms. They may also have

symptoms that suggest autoimmune disease and complement

deficiency may be an etiologic factor in the development of

autoimmune disease.

 

C3 is at the entry point for all 3 activation pathways to activate the

MAC. C3 deficiency may result in pneumococcal and neisserial

infections 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 of

cases have been described in which the protein is present but is

non functional. These rare cases require a functional assay to

detect the deficiency.

Reference Values

21-50 unit/mL

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 complement

values are consistent with a C3 deficiency.

Cautions

The total complement assay (#8167 "Complement, Total, Serum")

should be used as a screen for suspected complement deficiencies

before ordering individual complement component assays. A

deficiency of an individual component of the complement cascade

will result in an undetectable total complement level.

 

Absent (or low) C3 functional levels in the presence of normal C3

antigen levels should be replicated with a new serum specimen

to confirm that C3 inactivation did not occur during shipping.

Clinical Reference

1.   Davis ML, Austin C, Messmer BL, et al:  IFCC-standardization

      pediatric reference intervals for 10 serum proteins using the

      Beckman Array 360 system. Clin Biochem 1996;29(5):489-492

 

2.   Gaither TA, Frank MM:  Complement. In Clinical Diagnosis and

      Management by Laboratory Methods. 17th edition. Edited by

      JB Henry. Philadelphia, WB Saunders Company, 1984, pp 879-892

 

3.   O'Neil KM:  Complement deficiency. Clin Rev Allergy Immunol

      2000;19:83-108

 

4.   Frank MM:  Complement deficiencies. Pediatr Clin North Am

      2000;47(6):1339-1354

Key