Overview
C4 is a classical (and lectin) pathway component quantified in serum by nephelometry/immunoturbidimetry.
Clinical Importance
Pair with C3 in SLE and immune-complex evaluations
Investigate hereditary angioedema pathways only as an adjunct (C1-INH assays are primary)
Specimen Requirements
- Specimen: serum (preferred for most serology/autoimmune immunoassays) or plasma when the method is validated
- Tube: gold/SST (serum separator) or red-top serum; follow method-specific plasma acceptability
- Handling: allow SST to clot fully; centrifuge and separate promptly; avoid repeated freeze–thaw
- Storage: refrigerate short-term; freeze aliquots for delayed testing per manufacturer stability
- Stability: analyte- and kit-specific; do not exceed validated time/temperature limits
- Complement: process promptly; complement proteins are labile; follow SOP for delayed testing and freeze if required
Analytical Method
Immunochemical quantification of C4 protein concentration; not a functional C4 assay.
Instruments Used
Automated analyzers
- Roche Cobas immunoassay systems
- Abbott Alinity / Architect immunoassay systems
- Siemens Healthineers Atellica / ADVIA Centaur immunoassay systems
- Beckman Coulter Access / DxI immunoassay systems
- bioMérieux VIDAS / BioFire immunoassays (selected analytes)
- Ortho Clinical Diagnostics Vitros immunoassay systems
- DiaSorin LIAISON immunoassay systems
- Phadia / Thermo Fisher ImmunoCAP allergy systems
- Manual ELISA, CLIA, and immunofluorescence platforms
Manual methods
- Manual ELISA, agglutination, and immunofluorescence methods in selected settings
Reference Values
Assay cutoffs, units, and reference intervals are method- and manufacturer-specific. Always interpret against the performing laboratory report (CLSI EP28-A3c for quantitative intervals where applicable). Serologic patterns require clinical correlation and current guideline algorithms (WHO/CDC/society guidance as adopted locally).
Adult C4 is often roughly 0.1–0.4 g/L on many methods; verify the report interval.
Interpretation
Common Causes of Increased Results
- Elevated C4 may accompany acute-phase responses
Common Causes of Decreased Results
- Low C4; classical pathway consumption, genetic partial deficiency, or other causes; correlate with C3
Clinical Significance
- SLE and immune-complex disease panels
- Complement profiling with CH50
Factors Affecting Results
Pre-analytical
- Hemolysis, lipemia, or icterus (assay-dependent interference)
- Wrong tube type or incomplete clotting of SST
- Repeated freeze–thaw of serum aliquots
- Testing too early or too late relative to infection/vaccination timing for serology
Analytical
- Calibration/QC failure; reagent lot shifts
- Hook effect / antigen excess on selected immunoassays
- Heterophile antibodies, rheumatoid factor, or biotin interference (assay-dependent)
Post-analytical
- Applying another laboratory’s cutoff to a different method
- Interpreting a single serology result without clinical timing and confirmatory algorithms
Advantages and Limitations
Advantages
- Complements C3 interpretation
Limitations
- Partial C4 null alleles common; mild lows need context
Clinical Pearls
C4 completes the C3 story; order them together when complement consumption is the question.
References
Authoritative textbooks, guidelines, and reviews supporting this laboratory test reference. Verify reference intervals, critical limits, and method details against institutional protocols and current guideline editions.
Textbooks
- McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods (verify current edition). Chapters on clinical immunology and serology.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (verify current edition). Immunoassay and protein chapters.
CLSI and Professional Guidelines
- Centers for Disease Control and Prevention. Laboratory testing recommendations for infectious disease serology and related algorithms (verify current CDC guidance for the pathogen).
- Clinical and Laboratory Standards Institute. Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory. CLSI document EP28-A3c (verify current edition).
- Clinical and Laboratory Standards Institute. Immunoassay and related method evaluation documents (e.g., EP05, EP06, EP07, EP17; verify current editions relevant to the assay).
- College of American Pathologists. Accreditation and proficiency testing expectations for immunology/serology (verify current CAP checklist requirements).
- International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). Recommendations relevant to protein and immunoassay measurands (verify current IFCC documents).
WHO Publications
- World Health Organization. Laboratory and diagnostic guidance for serologic and immunologic testing (verify current WHO publication for the analyte/assay).
Frequently Asked Questions
Common questions about using this Complement C4 laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the Complement C4 laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of Complement C4. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find Complement C4 specimen requirements?
See the Specimen Requirements section on this page for specimen information related to Complement C4. Confirm collection details against your laboratory protocol.
Where are Complement C4 reference values listed?
See the Reference Values section on this Laboratory Library page. Reference ranges can vary by method and population, so verify intervals with your laboratory.
How do I interpret Complement C4 results?
Use the Interpretation section for laboratory interpretation framing, including increased and decreased result patterns when provided. Do not treat educational text as a patient-specific diagnosis.
What analytical method is used for Complement C4?
See the Analytical Method section for principle and method notes. Method details may differ by instrument and institutional SOP.
Are related laboratory tests linked from Complement C4?
Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.
How can I continue learning after reading Complement C4?
Use Continue learning links on this page for related Academy lessons, medical laboratory quizzes, or laboratory case studies when available for this Immunology & Serology topic.
What are the clinical pearls for Complement C4?
See the Clinical Pearls section for concise laboratory practice notes associated with Complement C4. Apply pearls alongside institutional protocols.
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