Test Overview
Clinical Utility
Complement proteins are components of the innate immune system. There are 3 pathways to complement activation: 1) the classical pathway, 2) the alternative (or properdin) pathway, and 3) the lectin (or mannan-binding lectin) pathway. The classical pathway of the complement system is composed of a series of proteins that are activated in response to the presence of immune complexes. A single IgM molecule or 2 IgG molecules are sufficient to trigger activation of the recognition complex initiated by C1q. The activation process triggers a cascade that includes an amplification loop. The amplification loop is mediated by C3, with cleavage of a series of proteins, and results in 3 main end products: 1) anaphylatoxins that promote inflammation (C3a, C5a), 2) opsonization peptides that are chemotactic for neutrophils (C3b) and facilitate phagocytosis, and 3) the membrane attack complex (MAC), which promotes cell lysis.
Patients with deficiencies of the late complement proteins (C5, C6, C7, C8, and C9) are unable to form the MAC and may have increased susceptibility to neisserial infections. Approximately 40 cases of C5 deficiency have been reported.
Complement levels can be detected by antigen assays that quantitate the amount of the protein. For most of the complement proteins, a small number of cases have been described in which the protein is present but is nonfunctional. These rare cases require a functional assay to detect the deficiency.
Method
Liposome immune lysis assay (LILA)
Analytical Platform
Siemens Advia XPT
Method Description
C5 complement activity is measured by mixing patient serum with a C5-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 C5. The target liposomes are a commercial reagent (WAKO total complement CH50), and the assay is performed on a Siemens Advia XPT.(Unpublished Mayo method)
Result Included
C5 Complement
Aliases/Synonyms
Complement Component 5, C5 activity
Specimen
Specimen Type
SerumC5 Complement testing is available as functional or antigen testing.
Non-interfaced clients should specify the Test ID C5FX S or "Functional" when ordering this test. Not doing so may cause processing delays while ICL ascertains which test you are requesting .
Antigen test: C5 Complement Antigen, Serum
Containers
Collection Containers
Red top (no additive)
Volume
Sample Volume
1 mL
Minimum Volume
0.5 mL
Patient Preparation
Fasting preferred
Collection & Handling
Handling Information
Immediately after specimen collection, place the tube on wet ice. Centrifuge and aliquot serum into plastic vial. Immediately freeze specimen. Store and send frozen.
Stability
| Ambient | Refrigerated | Frozen |
|---|---|---|
| Unacceptable | Unacceptable | 14 days |
Rejection Criteria
| Criteria | Specification |
|---|---|
| Lipemia | Gross |
Performance & Interpretation
Turnaround Time
8 days
Results
-
C5 Complement FunctionalU/mL≥ 39
Comment
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 C5 levels in the presence of normal C3 and C4 values are consistent with a C5 deficiency. Absent C5 levels in the presence of low C3 and C4 values suggest complement consumption.
Normal results indicate both normal C5 protein levels and normal functional activity.
Referral Location
Out-of-Country
Interface & Setup
HL7 Interface Codes
| Order Code | Result Name | Result Codes | Units |
|---|---|---|---|
| C5FX S | C5 Complement, Functional, S | 63662 | U/mL |
Test Version
Last Updated
2025-12-16