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Immunology & Serology

C-Reactive Protein (CRP)

C-Reactive Protein (CRP) is a Immunology & Serology laboratory test reference in the LabPedia Medical Laboratory Library. Acute-phase protein measured by immunoassay or immunoturbidimetry; nonspecific inflammation marker distinct from high-sensitivity CRP cardiovascular assays.

Also known asCRP; Serum CRP

Overview

CRP is a pentraxin acute-phase reactant synthesized by the liver in response to IL-6 and related cytokines; laboratory assays quantify circulating CRP concentration.

Clinical Importance

Detect and monitor systemic inflammation and infection-related acute-phase responses

Support serial monitoring of inflammatory disease activity when used with clinical findings

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

Analytical Method

Immunoturbidimetric/nephelometric or immunoassay methods bind CRP with specific antibodies and quantify the immune complex signal. Standard CRP and hs-CRP may use different calibration ranges.

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).

Typical published adult CRP is often <5 mg/L (or <0.5 mg/dL) on many standard assays; verify the report unit and method interval.

hs-CRP uses a different clinical frame (cardiovascular risk) and must not be interchanged with infection CRP interpretation.

Interpretation

Common Causes of Increased Results

  • Elevated CRP supports an acute-phase response (infection, inflammation, tissue injury); nonspecific

Common Causes of Decreased Results

  • Low/undetectable CRP argues against a major acute-phase response but does not exclude all disease

Clinical Significance

  • Inflammation workup adjunct
  • Serial monitoring in selected inflammatory conditions

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

  • Rapid, widely available
  • Rises and falls faster than ESR in many settings

Limitations

  • Nonspecific
  • Does not identify the cause of inflammation

Clinical Pearls

Order the CRP your clinical question needs; infection CRP and hs-CRP are not interchangeable conversations.

Related Laboratory Tests

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 C-Reactive Protein (CRP) laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.

What is the C-Reactive Protein (CRP) laboratory test reference used for?

This Medical Laboratory Library page summarizes the clinical importance of C-Reactive Protein (CRP). Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.

Where can I find C-Reactive Protein (CRP) specimen requirements?

See the Specimen Requirements section on this page for specimen information related to C-Reactive Protein (CRP). Confirm collection details against your laboratory protocol.

Where are C-Reactive Protein (CRP) 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 C-Reactive Protein (CRP) 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 C-Reactive Protein (CRP)?

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 C-Reactive Protein (CRP)?

Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.

How can I continue learning after reading C-Reactive Protein (CRP)?

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 C-Reactive Protein (CRP)?

See the Clinical Pearls section for concise laboratory practice notes associated with C-Reactive Protein (CRP). Apply pearls alongside institutional protocols.

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