When to order C-Reactive Protein (CRP) or Erythrocyte Sedimentation Rate (ESR)?

When to Order CRP or ESR

CRP should be ordered as the preferred inflammatory marker for acute inflammation, while ESR is more appropriate for monitoring chronic inflammatory conditions, and both tests should be ordered together when the clinical picture is unclear.

Clinical Indications for CRP

• Acute inflammatory conditions:

  • Suspected bacterial infections
  • Early detection of inflammatory flares
  • Monitoring response to antimicrobial therapy
  • Post-surgical inflammation monitoring

• Advantages of CRP:

  • More sensitive and specific for acute phase inflammation (sensitivity 73%, specificity 78%) 1
  • Rises within 4-6 hours of inflammatory stimulus
  • Returns to normal quickly with resolution (half-life of 19 hours)
  • Less affected by non-inflammatory factors than ESR

Clinical Indications for ESR

• Chronic inflammatory conditions:

  • Monitoring disease activity in rheumatoid arthritis
  • Assessment of temporal arteritis/giant cell arteritis
  • Monitoring systemic lupus erythematosus
  • Evaluation of unexplained weight loss or fever of unknown origin

• Advantages of ESR:

  • Better reflects chronic inflammation due to longer half-life of fibrinogen 2
  • More sensitive to immunoglobulins and rheumatoid factor 3
  • May better measure overall disease severity in certain conditions

When to Order Both Tests

• Order both CRP and ESR when:
  • Clinical picture is unclear or diagnosis is uncertain
  • Distinguishing between flare and infection in autoimmune conditions
  • Monitoring rheumatoid arthritis disease activity and radiographic progression 4
  • Evaluating fever in systemic lupus erythematosus patients (ESR:CRP ratio helps distinguish flare from infection) 5

Disease-Specific Recommendations

1. Rheumatoid Arthritis:

   • Both ESR and CRP are components of disease activity measures (DAS28-ESR or DAS28-CRP) 6
   • Regular monitoring recommended for disease activity assessment 6

2. Kawasaki Disease:

   • Measure both CRP and ESR in children with fever >5 days and 2-3 classic symptoms 6
   • If elevated, proceed with additional testing (serum albumin, transaminases, CBC, urinalysis)

3. Psoriatic Arthritis:

   • ESR/CRP elevation indicates active disease requiring DMARD therapy 6
   • Particularly important when multiple swollen joints or structural damage is present

4. Systemic Lupus Erythematosus:

   • ESR:CRP ratio >15 suggests disease flare rather than infection 5

5. Immune Checkpoint Inhibitor Toxicity:

   • Consider ESR/CRP testing for suspected inflammatory arthritis (grade 2/3) 6

Interpretation of Results

CRP	ESR	Clinical Interpretation
Elevated	Normal	Acute inflammation (early infection, early flare)
Normal	Elevated	Chronic inflammation, autoimmune diseases, or non-inflammatory factors affecting ESR
Elevated	Elevated	Active inflammation (both acute and chronic processes)
Normal	Normal	Low likelihood of significant inflammation [1]

Common Pitfalls to Avoid

1. Do not dismiss elevated ESR when CRP is normal - This pattern can occur in several serious conditions including vertebral osteomyelitis, tuberculosis, and certain autoimmune diseases 1

2. Do not treat the marker instead of the disease - CRP and ESR are indicators, not therapeutic targets 1

3. Avoid ordering both tests routinely - Choose the appropriate test based on clinical scenario to reduce unnecessary testing 7

4. Remember factors affecting ESR independent of inflammation:

   • Age (increases with age)
   • Gender (higher in females)
   • Anemia (increases ESR)
   • Pregnancy (increases ESR)
   • Hypergammaglobulinemia (increases ESR)

5. Consider discordance between ESR and CRP - Occurs in approximately 28% of cases and may provide additional diagnostic information 3

By selecting the appropriate inflammatory marker based on the clinical context and understanding their limitations, you can optimize diagnostic accuracy while minimizing unnecessary testing.