What do CRP (C-reactive protein) and ESR (erythrocyte sedimentation rate) blood tests check for?

What CRP and ESR Check For

CRP and ESR are blood tests that measure systemic inflammation in your body—CRP detects acute inflammation and responds rapidly to changes, while ESR reflects chronic inflammation and responds more slowly. 1, 2

Primary Function of Each Test

C-Reactive Protein (CRP)

• CRP is a direct acute-phase reactant that rises and falls rapidly with active inflammation, making it superior for diagnosing acute infections and monitoring rapid treatment responses 1
• CRP has a sensitivity of 100% and specificity of 72% for identifying sepsis in ICU patients at levels ≥1.5 ng/ml 1
• CRP is most useful for acute inflammatory conditions including urinary tract infections, gastrointestinal infections, pulmonary infections, bloodstream infections, myocardial infarction, and venous thromboembolism 1, 3
• CRP generally returns to baseline within 2 months after surgery under normal circumstances 1

Erythrocyte Sedimentation Rate (ESR)

• ESR is an indirect measure of fibrinogen, which has a much longer half-life than CRP, making ESR most valuable for monitoring chronic inflammatory conditions rather than initial diagnosis 1, 2
• ESR values are typically considered elevated when they exceed 20 mm/h in men and 30 mm/h in women 1
• ESR remains elevated longer than CRP after inflammation resolution, which explains why these tests sometimes show discordant results 1
• ESR is particularly useful for monitoring giant cell arteritis, polymyalgia rheumatica, and inflammatory arthritis 1

Clinical Applications by Disease Category

Rheumatologic Conditions

• Both ESR and CRP are incorporated into disease activity scores (DAS28-ESR and DAS28-CRP) for rheumatoid arthritis, though they are not interchangeable 1
• ESR >40 mm/h has 93.2% sensitivity for giant cell arteritis with a negative likelihood ratio of 0.18 1
• ESR should be measured every 1-3 months during active rheumatologic disease until remission, then every 3-6 months in remission 1

Infectious Diseases

• CRP is superior to ESR for diagnosing acute infections because it rises and falls more rapidly 1, 2
• For prosthetic joint infection, when at least 2 of 3 tests (ESR >27 mm/h, CRP >0.93 mg/L, fibrinogen >432 mg/dL) are abnormal, sensitivity is 93% and specificity is 100% 1
• Infections, particularly bone and joint infections, are more commonly associated with elevated ESR/low CRP discordance 4, 3

Cardiovascular Events

• Ischemic strokes or transient ischemic attacks almost invariably show elevated ESR/low CRP patterns 3
• Myocardial infarction and venous thromboembolism typically show elevated CRP/low ESR patterns 3

Inflammatory Bowel Disease

• CRP has sensitivity of 73% and specificity of 78% at a threshold of 5-6 mg/L for IBD, but guidelines recommend against using CRP alone to screen for IBD 5
• ESR has even lower sensitivity (54-78%) and specificity (46-95%) for IBD screening and should not be used for this purpose 5

Important Factors That Affect Test Results

Factors That Artificially Elevate ESR

• Anemia and azotemia (kidney dysfunction) can artificially elevate ESR values independent of inflammatory activity 1, 4
• Women typically have higher baseline ESR values than men 1
• ESR normally increases with age 1
• Low serum albumin is associated with elevated ESR 4

Factors That Affect CRP

• Low serum albumin can affect CRP measurements 4
• Renal insufficiency is associated with increased risk of elevated ESR/low CRP discordance 4

When Results Are Discordant

CRP/ESR discordance occurs in approximately 12% of patients (1 in 8), with results differing by 2 or more quartiles 3

High ESR with Low CRP Suggests:

• Chronic inflammatory conditions like systemic lupus erythematosus 3
• Ischemic stroke or transient ischemic attack 3
• Underlying infection (particularly bone and joint infections) 4, 3
• Renal insufficiency 4
• Low albumin states 4

High CRP with Low ESR Suggests:

• Acute infections (urinary tract, gastrointestinal, pulmonary, bloodstream) 3
• Acute cardiovascular events (myocardial infarction, venous thromboembolism) 3
• Early phase of inflammation before ESR rises 2

Critical Limitations

• Both ESR and CRP lack sensitivity and specificity, so these tests should only be used in combination with clinical history and physical examination 6
• Normal inflammatory markers do not exclude inflammatory disease, and clinical judgment remains essential when suspicion is high 5
• ESR as a measure of inflammation may be limited in settings of infection, renal insufficiency, and low albumin 4