Unit Converter
C-Reactive protein (CRP)
(Acute-Phase Protein – Marker of Inflammation, Infection & Cardiovascular Risk)
Synonyms
- CRP
- C-reactive protein
- High-sensitivity CRP (hs-CRP)
- Acute-phase reactant
- Inflammatory marker
Units of Measurement
- nmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
- g/L
Description
C-reactive protein is a pentameric acute-phase protein produced by the liver in response to inflammation, infection, or tissue injury.
Stimulated primarily by:
- IL-6
- IL-1β
- TNF-α
CRP rises rapidly within 6–8 hours, peaks at 48 hours, and falls quickly with resolution, making it a highly sensitive marker of:
- Acute bacterial infection
- Inflammation (autoimmune, rheumatic)
- Tissue injury / necrosis
- Postoperative states
- CVD risk assessment (hs-CRP)
Physiological Role
- Binds phosphocholine on microbes → promotes opsonization
- Activates classical complement pathway
- Enhances phagocytosis
- Marker of systemic inflammation
- Plays role in atherosclerosis (low-grade inflammation)
Clinical Significance
Elevated CRP
1. Acute Bacterial Infections (Highest Levels)
- CRP >100 mg/L strongly suggests bacterial etiology
- Sepsis, pneumonia, pyelonephritis
2. Inflammatory & Autoimmune Diseases
- Rheumatoid arthritis
- Vasculitis
- Inflammatory bowel disease (IBD)
- SLE (less pronounced)
3. Cardiovascular Disease (hs-CRP)
Low-grade inflammation marker:
- <1 mg/L → Low risk
- 1–3 mg/L → Moderate risk
- 3 mg/L → High risk of CVD events
4. Postoperative or Trauma Response
CRP rise is normal; failure to decline → infection/complication.
5. Chronic Conditions
- Obesity
- Metabolic syndrome
- Diabetes
- Chronic kidney disease
6. Cancer
Moderate elevation in:
- Lymphoma
- Solid tumors
- Advanced malignancies
Low CRP
Normal (<3 mg/L).
Low CRP is not clinically significant unless immune suppression is suspected.
Reference Intervals
(Tietz 8E + AHA/ACC + CDC + Mayo/ARUP)
Standard CRP
- < 5 mg/L (normal)
High-Sensitivity CRP (Cardiac risk)
- < 1 mg/L → Low CVD risk
- 1–3 mg/L → Moderate risk
- > 3 mg/L → High CVD risk
- > 10 mg/L → Not for risk stratification → indicates acute inflammation
Infections
- 10–40 mg/L → mild inflammation
- 40–200 mg/L → significant bacterial infection
- > 200 mg/L → severe sepsis / extensive inflammation
Unit Meanings
| Unit | Meaning |
| nmol/L | nanomoles per liter |
| mg/L | milligrams per liter |
| mg/dL | milligrams per deciliter |
| mg% / mg/100mL | identical to mg/dL |
| µg/mL | micrograms per milliliter |
| g/L | grams per liter |
Diagnostic Uses
1. Diagnosis & Monitoring of Infection
CRP correlates with:
- Treatment response
- Severity of bacterial infection
- Post-surgical complications
2. Autoimmune & Inflammatory Disorders
Tracks disease activity:
- RA flare → ↑ CRP
- IBD (Crohn > Ulcerative colitis)
- Vasculitis
3. Cardiovascular Risk (hs-CRP)
Adds predictive value to lipid panels.
4. Neonatal Sepsis Screening
CRP rises within 12 hours of infection.
5. Cancer / Chronic Disease Inflammation
6. Monitoring Therapy
- Antibiotics
- Steroids
- Immunomodulators
- Biologics (e.g., anti-TNF)
Analytical Notes
- Serum or plasma acceptable.
- hs-CRP uses high-sensitivity immunoassay (detects down to 0.1 mg/L).
- Standard CRP detects ≥5 mg/L.
- Hemolysis/lipemia minimal effect.
- CRP rises faster than ESR and falls more quickly.
Clinical Pearls
- CRP >100 mg/L almost always indicates bacterial infection, not viral.
- hs-CRP should not be measured during acute illness (values unreliable).
- ESR is slow to change; CRP is rapid and more specific.
- Severe obesity increases baseline CRP.
- In SLE, CRP can remain normal even during active flare.
Interesting Fact
CRP was named because it reacts with the C-polysaccharide of Streptococcus pneumoniae - the first acute-phase protein ever discovered.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Acute-Phase Reactants.
- AHA/ACC - Cardiovascular Risk Guidelines (hs-CRP).
- CDC/IFCC - CRP Standardization.
- Mayo Clinic Laboratories - CRP & hs-CRP.
- ARUP Consult - Inflammation & Acute Phase Reactants.
- MedlinePlus / NIH - CRP Test.
- Rheumatology Texts - CRP in Autoimmune Diseases.