C-Reactive Protein
CRP· also: hs-CRP, hsCRP, C-Reactive Protein
Clinical Overview
CRP is an acute-phase protein produced by the liver in response to inflammation, infection, and tissue injury. It rises within 6–12 hours of an inflammatory stimulus and falls quickly when inflammation resolves. CRP is more specific and sensitive than ESR for detecting acute inflammation.
Why This Test Matters
CRP is widely used to detect and monitor acute infections, guide antibiotic use, and monitor treatment of inflammatory diseases. High-sensitivity CRP (hsCRP) at lower levels (1–3 mg/L) predicts cardiovascular risk in apparently healthy individuals. Markedly elevated CRP (>100 mg/L) almost always indicates bacterial infection or significant tissue injury.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 0 – 10 | mg/L | Standard CRP; most labs use <5–10 mg/L |
| 0 – 3 | mg/L | High-sensitivity CRP (hsCRP) for cardiovascular risk |
Also reported in: mg/dL.
Critical (Panic) Values
Critical High: > 200 mg/L. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High CRP Causes
- Bacterial infections (most potent stimulus — CRP often >100 mg/L)
- Tissue injury: surgery, trauma, burns, myocardial infarction
- Autoimmune diseases: rheumatoid arthritis, lupus, vasculitis
- Inflammatory bowel disease
- Malignancy
- Pancreatitis
Low CRP Causes
- Not clinically significant (low CRP is normal)
- Statin therapy (reduces CRP as a beneficial cardiovascular effect)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required for standard CRP. For hsCRP (cardiovascular risk assessment), fasting for 9–12 hours improves accuracy. Avoid vigorous exercise 24 hours before testing as it can transiently elevate CRP.
Factors That Can Affect Results
- Obesity (chronically elevated CRP)
- Smoking (elevates CRP)
- Statin therapy (lowers CRP)
- Oral contraceptives (raise CRP)
- Exercise (transiently raises CRP)
- Pregnancy (CRP rises modestly)
Related Topics
Frequently Asked Questions
Does a normal CRP rule out infection?
A normal CRP makes significant bacterial infection less likely but does not rule it out entirely. CRP takes 6–12 hours to rise significantly, so a very early infection may have a normal CRP. Viral infections cause variable CRP elevations — usually lower than bacterial infections. Procalcitonin is a more specific marker for bacterial infections and is often used alongside CRP.
What is high-sensitivity CRP and why does it matter?
Standard CRP detects acute inflammation at levels above 5–10 mg/L. High-sensitivity CRP (hsCRP) measures much lower concentrations and is used for cardiovascular risk stratification in healthy people. An hsCRP above 3 mg/L is associated with doubled cardiovascular risk. The JUPITER trial showed that statin therapy reduces hsCRP and cardiovascular events even in people with normal LDL cholesterol.
What CRP level indicates severe sepsis?
While there is no single definitive threshold, CRP above 100 mg/L almost always indicates serious bacterial infection or significant tissue injury. In sepsis, CRP can rise to 200–400 mg/L. Serial measurements are more useful than a single value — rising CRP despite antibiotics suggests inadequate treatment or a complication such as abscess formation.