Overview
C3 is the central complement component measured by nephelometry/immunoturbidimetry as a concentration in serum.
Clinical Importance
Evaluate complement consumption in SLE and immune-complex disease
Support complement deficiency workups with CH50/C4
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
Antisera to C3 form immune complexes measured by light scatter or turbidimetry. Results in g/L or mg/dL.
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 C3 is often roughly 0.9–1.8 g/L on many methods; use the report interval. Acute-phase elevation can raise C3.
Interpretation
Common Causes of Increased Results
- Elevated C3; acute-phase response
Common Causes of Decreased Results
- Low C3; complement consumption or deficiency; interpret with C4 (low C3+C4 suggests classical pathway consumption)
Clinical Significance
- SLE activity adjunct laboratory panels
- Glomerulonephritis complement profiles
Factors Affecting Results
Pre-analytical
- Delayed processing (less critical than CH50 but still follow SOP)
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
- Standard, quantitative, widely available
Limitations
- Nonspecific
- Normal C3 does not exclude all complement disease
Clinical Pearls
Low C3 with normal C4 points alternative pathway; low C3 and C4 points classical/immune-complex consumption.
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 C3 laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the Complement C3 laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of Complement C3. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find Complement C3 specimen requirements?
See the Specimen Requirements section on this page for specimen information related to Complement C3. Confirm collection details against your laboratory protocol.
Where are Complement C3 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 C3 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 C3?
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 C3?
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 C3?
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 C3?
See the Clinical Pearls section for concise laboratory practice notes associated with Complement C3. Apply pearls alongside institutional protocols.
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