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Crp

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Here is a thorough overview of C-Reactive Protein (CRP) drawn from multiple authoritative textbooks:

C-Reactive Protein (CRP)

History & Structure

CRP was first isolated in 1930 from plasma of patients with pneumococcal pneumonia. It was named because it binds to the C-polysaccharide of Streptococcus pneumoniae. It belongs to the pentraxin family - a 115 kDa pentamer of five non-covalently linked 23 kDa subunits arranged in cyclic symmetry. With Ca²⁺ ions, it binds phosphocholine, phospholipids, fibronectin, chromatin, and histones - all exposed at sites of tissue damage. This structure has been highly conserved over hundreds of millions of years of evolution. - Firestein & Kelley's Textbook of Rheumatology

Synthesis & Kinetics

  • Produced primarily by hepatocytes, stimulated by cytokines - chiefly interleukin-6
  • Rises within 6-12 hours of the initial inflammatory insult
  • Peaks at 2-3 days, reflecting the extent of tissue injury
  • Half-life ~19 hours - drops rapidly once the stimulus is removed
  • Levels may be affected by underlying liver dysfunction (single-organ origin)
  • Persistent elevations are seen in chronic inflammatory/infectious states (e.g., active RA, pulmonary TB) and extensive malignancy
  • Tietz Textbook of Laboratory Medicine, 7th Edition & Firestein & Kelley's Textbook of Rheumatology

Functions

CRP has both pro- and anti-inflammatory roles:
Pro-inflammatory:
  • Activates the classical complement pathway
  • Interacts with immune cells via Fcγ receptors
  • Induces inflammatory cytokines and tissue factors
  • Sheds IL-6 receptor, leading to complement-dependent tissue damage
Anti-inflammatory:
  • Promotes non-inflammatory clearance of apoptotic cells
  • Prevents neutrophil adhesion to the endothelium
  • Opsonizes particles for phagocytosis
  • Firestein & Kelley's Textbook of Rheumatology

Normal Values & Interpretation

LevelInterpretation
Median ~1 mg/L (normal population)Normal baseline
<1 mg/dL (10 mg/L)Normal or minor elevation
1-10 mg/dLModerate elevation
>10 mg/dLMarked elevation
Up to 300+ mg/LAcute severe illness
Most healthy adults have levels <0.3 mg/dL. Concentrations >1 mg/dL reflect clinically significant inflammatory disease. - Henry's Clinical Diagnosis and Management by Laboratory Methods

hsCRP for Cardiovascular Risk (AHA/CDC Joint Recommendations):

  • <1 mg/L - Low cardiovascular risk
  • 1-3 mg/L - Intermediate cardiovascular risk
  • >3 mg/L - High cardiovascular risk
  • Results should ideally be averaged from two specimens ~2 weeks apart
  • If >10 mg/L: search for infection/inflammation, discard result, retest in 2 weeks

Conditions Associated With Elevated CRP

Degree of ElevationConditions
Normal/Minor (<1 mg/dL)Vigorous exercise, common cold, pregnancy, gingivitis, seizures, depression, insulin resistance/diabetes, obesity
Moderate (1-10 mg/dL)MI, malignancies, pancreatitis, mucosal infections (bronchitis, cystitis), most systemic autoimmune diseases, rheumatoid arthritis
Marked (>10 mg/dL)Acute bacterial infection (80-85%), major trauma, systemic vasculitis
  • Firestein & Kelley's Textbook of Rheumatology

CRP Levels Across Rheumatic Diseases:

Range of CRP levels in rheumatic diseases
Note: SLE without serositis characteristically shows only modest CRP elevation - a very useful clinical pearl. Vasculitis and Adult Still's disease can push CRP to 50+ mg/dL.

CRP vs. ESR

FeatureESRCRP
Response timeSlowRapid
Affected by age/sexYesRelatively unaffected
Affected by anemia/polycythemiaYesNo
ReproducibilityLowerHigh
Requires fresh sampleYesCan use stored sera
DisadvantageReflects many proteinsNot sensitive to SLE disease activity
  • Firestein & Kelley's Textbook of Rheumatology

CRP in Infection

  • Estimated pooled sensitivity 75%, specificity 67% for differentiating bacterial from non-bacterial inflammation
  • CRP >500 mg/L: bacterial infection present in ~88% of cases
  • Used to guide antibiotic therapy decisions (e.g., COPD exacerbations, bone & joint infections in children)
  • CRP-guided antibiotic discontinuation (discontinue at 75% reduction from peak): shown to be non-inferior to fixed 7- or 14-day courses in gram-negative bacteremia
  • CRP improves with appropriate antibiotic therapy; persistently elevated levels associated with poor outcomes
  • Tietz Textbook of Laboratory Medicine, 7th Edition

CRP and Cardiovascular Disease

  • Higher baseline CRP is associated with increased risk of ASCVD and stroke
  • The Physicians' Health Study showed CRP averaged 1.51 mg/L in MI patients vs. 1.13 mg/L in controls
  • The 2019 ACC/AHA primary prevention guideline identifies CRP ≥2.0 mg/L as an "ASCVD risk enhancer" that may influence therapeutic decisions
  • Universal hs-CRP screening is not recommended
  • Most useful in patients with intermediate 10-year risk (10-20%) where additional information may guide therapy
  • Henry's Clinical Diagnosis and Management by Laboratory Methods

Measurement

  • Measured by immunoassay or laser nephelometry
  • High-sensitivity CRP (hs-CRP) methods are needed to accurately measure concentrations in the 0.3-1 mg/dL range
  • No uniform reporting exists across labs (values may be in mg/L, μg/mL, or mg/dL - be careful with units)
  • Population differences exist across sex and racial-ethnic groups in the US
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