What does a C-Reactive Protein (CRP) level evaluate?

C-Reactive Protein (CRP): A Marker of Inflammation

C-reactive protein (CRP) is a sensitive biomarker of inflammation that evaluates both acute inflammatory responses and chronic inflammatory states in the body. 1 As an acute-phase protein produced by the liver in response to inflammatory cytokines, CRP serves as an objective marker of inflammation whose levels can increase dramatically during infection, tissue injury, or other inflammatory conditions.

Primary Functions and Clinical Significance

CRP functions as:

• Marker of acute inflammation: Levels can rise several hundredfold in response to acute injury, infection, or other inflammatory stimuli 2
• Indicator of chronic inflammation: Used to assess low-grade, persistent inflammatory states 2
• Component of innate immunity: Acts as a mediator in the immune response 3

Interpretation of CRP Levels

Normal and Elevated Ranges

• Normal CRP levels: Typically below 3 mg/L in healthy individuals 2
• Risk stratification categories (for cardiovascular risk assessment):
  • Low risk: <1.0 mg/L
  • Average risk: 1.0-3.0 mg/L
  • High risk: >3.0 mg/L 2
• Acute inflammation threshold: Values ≥10 mg/L traditionally suggest acute inflammatory processes 2
  • Mild inflammation: 10-40 mg/L
  • Acute inflammation/bacterial infection: 40-200 mg/L 2
• Extreme elevations: CRP can rise above 500 mg/L during severe acute illness 2

Clinical Applications

1. Cardiovascular disease risk assessment:

   • hsCRP (high-sensitivity CRP) may be used at the physician's discretion in patients at intermediate risk (10-20% risk of CHD per 10 years) 2
   • Can help direct further evaluation and therapy in primary prevention 2

2. Monitoring inflammatory conditions:

   • Useful in monitoring disease activity in conditions like Crohn's disease 4
   • Less reliable marker in ulcerative colitis 4

3. Infection detection and monitoring:

   • Helps differentiate between infectious and non-infectious causes of inflammation 2
   • Valuable for monitoring response to antibiotic treatment 5

4. Prognostic value:

   • Elevated levels associated with increased risk of colorectal cancer 4
   • Predictor of mortality in certain conditions 2

Important Considerations and Limitations

• Non-specificity: CRP is not specific to any single disease state 1

• Interpretation challenges: Values >10 mg/L are not always indicative of acute infection/injury and may reflect other factors 2

• Confounding factors that can elevate CRP:

  • Obesity
  • Physical inactivity
  • Smoking 6
  • Inflammatory bowel disease
  • Rheumatoid arthritis
  • Long-term alcoholism 2

• Measurement recommendations:

  • For stable patients, two measurements (optimally 2 weeks apart) should be averaged 2
  • If CRP >10 mg/L, testing should be repeated and patient examined for sources of infection or inflammation 2

Clinical Practice Guidelines

• Serial testing of CRP should not be used to monitor treatment effects 2
• Patients with persistently unexplained marked elevation of CRP (>10 mg/L) after repeated testing should be evaluated for non-cardiovascular causes 2
• CRP results should be expressed in mg/L to one decimal point 2

Common Pitfalls

1. Over-reliance on the 10 mg/L threshold: This cutoff originated from older studies with less sensitive assays and may inadvertently exclude individuals of interest in research or clinical assessment 2

2. Misinterpreting isolated values: Single measurements may be less valuable than sequential measurements in diagnosing infection 2

3. Assuming specificity: While sensitive, CRP lacks specificity for particular diseases and should be interpreted in clinical context 1

4. Ignoring lifestyle factors: CRP levels can be influenced by lifestyle factors like diet, exercise, and smoking, which should be considered in interpretation 6

CRP remains a valuable tool in clinical practice when used appropriately and interpreted in the context of the patient's overall clinical presentation and other diagnostic findings.