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Test Code C4FX C4 Complement, Functional, Serum

Reporting Name

C4 Complement, Functional, S

Useful For

Diagnosis of C4 deficiency


Investigation of a patient with an undetectable total complement (CH50) level

Method Name

Automated Liposome Lysis Assay

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type

Serum Red

Specimen Required

Collection Container/Tube: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL

Collection Instructions:

1. Immediately after drawing the specimen, place the tube on wet ice.

2. Spin down and separate serum from clot.

3. Immediately freeze specimen.

Additional Information: Fasting preferred.

Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time
Serum Red Frozen 14 days

Reject Due To


Mild OK; Gross OK


Mild OK; Gross reject


Mild OK; Gross OK


Serum gel tube

Reference Values

22-45 U/mL

Day(s) and Time(s) Performed

Monday through Saturday; Continuous with a 3 p.m. cutoff

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
C4FX C4 Complement, Functional, S In Process


Result ID Test Result Name Result LOINC Value
C4FX C4 Complement, Functional, S In Process

Analytic Time

Same day/1 day


The total complement assay (COM / 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) C4 functional levels in the presence of normal C4 antigen levels should be replicated with a new serum specimen to confirm that C4 inactivation did not occur during shipping.

Method Description

C4 complement activity is measured by mixing patient serum with a C4-deficient serum. The lytic activity of the serum mixture is tested against sensitized, labeled liposomes. If lysis occurs, the patient serum must be the source of the C4. The target liposomes are a commercial reagent (WAKO total complement CH50), and the assay is performed on a Hitachi 911.(Unpublished Mayo information Yamamoto S, Kubotsu K, Masaaki K, et al: Automated homogeneous liposome-based assay system for total complement activity. Clin Chem 1995;41:586-590)


Low levels of complement may be due to inherited deficiencies, acquired deficiencies, or due to complement consumption (eg, as a consequence of infectious or autoimmune processes).


Absent C4 levels in the presence of normal C3 and C2 values are consistent with a C4 deficiency.


Normal results indicate both normal C4 protein levels and normal functional activity.


In hereditary angioedema, a disorder caused by C1 esterase inhibitor deficiency, absent or low C4 and C2 values are seen in the presence of normal C3 (due to activation and consumption of C4 and C2).

Specimen Retention Time

14 days

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

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 generate the peptides that are necessary clear immune complexes and to attract neutrophils or to generate to 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.


Approximately 20 cases of C4 deficiency have been reported. Most of these patients have systemic lupus erythematosus (SLE) or glomerulonephritis. Patients with C4 deficiency may also have frequent bacterial infections.


Complement levels can be detected by antigen assays that quantitate the amount of the protein (C4 / Complement C4, 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.