What is the practical use of Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP) in clinical practice?

Practical Use of ESR and CRP in Clinical Practice

Primary Recommendation

CRP should be prioritized over ESR for diagnosing acute inflammatory conditions due to its superior analyte characteristics, rapid response to inflammation, and better correlation with disease activity, while ESR remains valuable for monitoring chronic inflammatory diseases and specific conditions like giant cell arteritis and polymyalgia rheumatica. 1, 2

When to Use CRP

CRP is the preferred inflammatory marker for:

• Acute infections and inflammatory conditions - CRP rises and falls rapidly with active inflammation, making it superior for diagnosis and monitoring treatment response in acute settings 1, 3
• Cardiovascular risk assessment - Obtain 2 measurements (fasting or nonfasting) 2 weeks apart and average the results in metabolically stable patients 1
• Inflammatory bowel disease - CRP correlates better with endoscopic activity in Crohn's disease than ESR, with a clinically relevant cutoff of 5 mg/L 4
• Ruling out active inflammation - Normal CRP (<5 mg/L) combined with low fecal calprotectin effectively excludes active inflammation with 81% sensitivity 4
• Predicting biologic therapy response - Elevated baseline CRP predicts better response to anti-TNF therapy 4

CRP measurement guidelines:

• Express results only as mg/L to 1 decimal point 1
• Risk categories: low risk <1.0 mg/L, average risk 1.0-3.0 mg/L, high risk >3.0 mg/L 1
• If CRP >10 mg/L, repeat the test and examine for infection or inflammation sources 1

When to Use ESR

ESR is specifically indicated for:

• Giant cell arteritis (GCA) diagnosis - ESR >40 mm/h has 93.2% sensitivity with negative likelihood ratio of 0.18; ESR >100 mm/h has 92.2% specificity with positive likelihood ratio of 3.11 2
• Polymyalgia rheumatica (PMR) monitoring - ESR >40 mm/h predicts higher relapse rates; monitor every 1-3 months during active disease, then every 3-6 months in remission 2, 5
• Rheumatoid arthritis disease activity - ESR contributes 15% of DAS28-ESR calculations; measure every 1-3 months until remission, then every 3-6 months 2, 5
• Osteomyelitis in diabetic foot infections - ESR ≥70 mm/h has 81% sensitivity and 80% specificity 2, 5
• Prosthetic joint infection - ESR combined with CRP and fibrinogen achieves 93% sensitivity and 100% specificity when at least 2 of 3 tests are abnormal 2, 5
• Kawasaki disease monitoring - ESR often >40 mm/h and commonly ≥100 mm/h 2

ESR interpretation thresholds:

• Elevated: >20 mm/h in men, >30 mm/h in women 2, 5
• Moderate elevation: 50-100 mm/h indicates greater likelihood of significant disease 2
• Women have higher baseline values than men, and ESR increases with age 2, 5

Critical Interpretation Caveats

Factors that artificially elevate ESR without true inflammation:

• Anemia and azotemia 2, 5
• Elevated immunoglobulins and rheumatoid factor 2, 5
• Female gender and advanced age 2, 5

Common causes of CRP/ESR discordance (occurs in 12% of patients): 6

High CRP with normal ESR (6% of cases):

• Acute infections (urinary tract, gastrointestinal, pulmonary, bloodstream) 6
• Myocardial infarction or venous thromboembolism 6
• Early acute inflammatory conditions before ESR rises 3

High ESR with normal CRP (6% of cases):

• Resolving inflammatory disorders (32% of cases) - ESR remains elevated longer after inflammation resolves 7, 6
• Bone and joint infections 6
• Connective tissue diseases like systemic lupus erythematosus 6
• Ischemic stroke or transient ischemic attack 6
• False-positive ESR from anemia, renal disease, or other confounders (28% of cases) 7

Practical Algorithm for Clinical Use

For suspected acute inflammation or infection:

1. Order CRP as the primary test 1, 7
2. CRP provides faster, more accurate assessment of acute inflammatory activity 3, 7
3. Joint measurement of ESR and CRP is unwarranted in unselected hospital patients due to poor agreement (kappa=0.38) 7

For chronic inflammatory disease monitoring:

1. Use ESR for conditions like rheumatoid arthritis, PMR, and GCA 2, 5, 3
2. Measure every 1-3 months during active disease 2, 5
3. Reduce to every 3-6 months once remission achieved 2, 5
4. Rising ESR signals potential disease reactivation requiring clinical reassessment 5

For specific diagnostic scenarios:

• Suspected GCA with new headache, jaw claudication, or visual symptoms: Order ESR urgently; if >40 mm/h, refer immediately to specialist 2
• Inflammatory bowel disease: Use CRP (cutoff 5 mg/L) for disease activity assessment 4
• Diabetic foot infection with suspected osteomyelitis: ESR ≥70 mm/h supports diagnosis 2, 5
• Cardiovascular risk stratification: Use CRP with two measurements averaged 1

When encountering discordant results:

• High CRP/normal ESR: Prioritize acute infection workup, consider acute vascular events 6
• High ESR/normal CRP: Consider resolving inflammation, chronic conditions (SLE, stroke), or ESR confounders (anemia, renal disease) 7, 6
• In hospital practice, when inflammatory disorder is suspected, prioritize CRP over ESR due to ESR's slow variation and frequent confounding 7

Key Limitations

Neither test adequately discriminates between active and inactive disease in unselected populations - both lack sufficient sensitivity and specificity when used in isolation 8. Always interpret ESR and CRP in combination with clinical history and physical examination findings, never as standalone diagnostic tests 3, 9.