What are Sed(erythrocyte sedimentation) rate and CRP(C-reactive protein) used for?

Uses of Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP)

ESR and CRP are essential inflammatory biomarkers used to detect, diagnose, and monitor inflammatory conditions, with CRP being more responsive to acute inflammation while ESR better reflects chronic inflammatory states.

Primary Clinical Applications

Detection of Inflammation

• Both ESR and CRP serve as non-specific markers of systemic inflammation 1
• Used to differentiate organic from functional disease with similar sensitivity and specificity:
  • ESR: sensitivity 0.54-0.78, specificity 0.46-0.95
  • CRP: sensitivity 0.73, specificity 0.78 1

Diagnostic Applications

1. Inflammatory Bowel Disease (IBD)

   • Mandatory laboratory tests in suspected IBD include ESR and CRP 1, 2
   • Help distinguish IBD from functional disorders like IBS 1
   • CRP correlates better with endoscopic disease activity in Crohn's disease than in ulcerative colitis 1

2. Rheumatologic Conditions

   • Essential in evaluation of undifferentiated peripheral inflammatory arthritis (UPIA) 1
   • Help predict progression to rheumatoid arthritis 1
   • In rheumatoid arthritis, CRP better reflects acute inflammation while ESR may better measure general disease severity due to sensitivity to immunoglobulins 3

3. Prosthetic Joint Infection (PJI)

   • The combination of abnormal ESR and CRP provides the best sensitivity and specificity for detecting PJI 1
   • Should be performed in all patients with suspected PJI when diagnosis is not clinically evident 1

4. Autoinflammatory Diseases

   • Used to monitor disease activity in IL-1 mediated autoinflammatory diseases 1
   • Normalization of acute phase reactants is required to determine adequate dosing of IL-1 blocking treatment 1

Monitoring Disease Activity

• Used to assess treatment response and disease progression 1, 2
• CRP is more useful for monitoring acute inflammatory conditions due to shorter half-life 4
• ESR is better for monitoring chronic inflammatory conditions due to longer half-life of fibrinogen 4

Physiological Differences and Interpretation

Key Differences

• Response time:

  • CRP rises within 4-6 hours and peaks at 36-50 hours
  • ESR changes more slowly, taking days to rise and normalize 4, 5

• Half-life:

  • CRP: 19 hours (faster response to treatment changes)
  • ESR: indirect measure of fibrinogen with much longer half-life 4

• Influencing factors:

  • ESR affected by age, gender, anemia, immunoglobulins, and rheumatoid factor 3
  • CRP less affected by these non-inflammatory factors 5, 6

Interpretation of Results

Pattern	Interpretation
Elevated CRP, Normal ESR	Acute inflammation (more reliable) [6]
Normal CRP, Elevated ESR	Resolving inflammation, non-inflammatory factors affecting ESR, or false-positive ESR [6]
Elevated CRP, Elevated ESR	Active inflammation [2]
Normal CRP, Normal ESR	Low probability of active inflammation [2]

Clinical Pearls and Pitfalls

• Discordance between ESR and CRP occurs in approximately 33% of cases 6
• When discordant, CRP is generally more reliable for detecting active inflammation 3, 6
• In elderly patients, both tests have limited utility with low true-positive/false-positive ratios 7
• Neither test alone is sensitive or specific enough for diagnosis; they should be used in conjunction with clinical findings 5
• Fecal calprotectin is a more specific marker for intestinal inflammation than ESR or CRP in suspected IBD 2
• Always consider testing for specific infectious causes (e.g., C. difficile) when evaluating inflammatory conditions 1

Recommended Testing Approach

1. Order both tests initially when evaluating suspected inflammatory conditions
2. For acute conditions or monitoring treatment response, prioritize CRP
3. For chronic inflammatory conditions, consider both ESR and CRP
4. When results are discordant, generally favor CRP interpretation
5. Consider disease-specific biomarkers (e.g., fecal calprotectin for IBD) as appropriate