C-reactive Protein (CRP): Measurement and Clinical Significance
C-reactive protein (CRP) is an acute-phase protein synthesized by the liver in response to inflammatory cytokines that measures systemic inflammation in the body. 1, 2 It serves as a sensitive but non-specific biomarker that can detect inflammation from various causes including infection, tissue injury, and chronic inflammatory conditions.
Biological Characteristics of CRP
- Production timeline: CRP secretion begins 4-6 hours after an inflammatory stimulus, doubles approximately every 8 hours, and reaches peak concentration at 36-50 hours 2
- Detection window: CRP can detect inflammation for approximately 7-10 days after the inflammatory stimulus has resolved 2
- Structure: CRP exists in conformationally distinct forms:
- Native pentameric CRP (pCRP) - the primary circulating form
- Monomeric CRP (mCRP) - formed when the pentamer dissociates 3
Clinical Interpretation of CRP Levels
The American Heart Association categorizes CRP levels as follows 1, 2:
| CRP Level | Clinical Significance |
|---|---|
| <1.0 mg/L | Low cardiovascular risk |
| 1.0-3.0 mg/L | Average cardiovascular risk |
| >3.0 mg/L | High cardiovascular risk |
| 10-40 mg/L | Mild inflammation |
| 40-200 mg/L | Acute inflammation/bacterial infection |
| >500 mg/L | Severe acute illness |
Clinical Applications of CRP Measurement
- Cardiovascular risk assessment: High-sensitivity CRP (hsCRP) >3 mg/L indicates increased cardiovascular risk 2
- Infection monitoring: Helps distinguish bacterial infections from viral causes and monitors response to antibiotics
- Inflammatory disease monitoring: Used to track disease activity in conditions like rheumatoid arthritis and inflammatory bowel disease 2
- Surgical patients: Helps identify postoperative complications; infectious complications after major abdominal surgery are unlikely in patients with CRP below 159 mg/L on postoperative day 3 2
- Dialysis patients: Elevated CRP predicts all-cause and cardiovascular mortality in both hemodialysis and peritoneal dialysis patients 1, 2
Important Considerations for CRP Testing
- Measurement recommendations: For stable patients, two measurements (fasting or non-fasting) should be taken approximately two weeks apart and averaged 1
- Elevated levels: If CRP level is ≥10 mg/L, the test should be repeated and the patient examined for sources of infection or inflammation 1
- Reporting: CRP results should be expressed in milligrams per liter (mg/L) to one decimal point 1
- Limitations:
Factors Affecting CRP Levels
- Medications: NSAIDs and certain other drugs can affect CRP levels 2
- Comorbidities: Conditions like obesity, diabetes, and chronic kidney disease can elevate baseline CRP 2
- Age and gender: Slight variations may occur based on these factors
Biological Functions of CRP
CRP plays important roles in:
- Activating complement through the classical pathway
- Binding to Fc receptors on immune cells
- Acting as an opsonin for various pathogens
- Pattern recognition of altered self and foreign molecules
- Modulating both innate and adaptive immune responses 4, 5
CRP is not merely a marker but also a mediator in the inflammatory process, with different isoforms (pCRP vs. mCRP) potentially having distinct biological effects on inflammation 3.