C-Reactive Protein (CRP)
CRP is a liver-made acute-phase protein that rises within hours of any inflammatory stimulus. The most useful general marker of "something is inflamed".
What it measures
The liver releases CRP in response to interleukin-6 signalling from the inflammation site. CRP rises within 6 hours, doubles every 8 hours, and peaks at 36–50 hours. It falls as the trigger resolves with a half-life of ~19 hours. This responsiveness makes CRP a far better real-time inflammation tracker than ESR (which lags by days). High-sensitivity CRP (hsCRP) is the same molecule measured with a more sensitive assay, used for cardiovascular risk stratification at levels below 10 mg/L where standard CRP can't distinguish.
What a high value can mean
- Bacterial infection — typically pushes CRP above 100 mg/L; viral infections rarely above 40.
- Active autoimmune disease — rheumatoid arthritis, vasculitis, inflammatory bowel disease.
- Tissue injury — myocardial infarction, surgery, burns, trauma.
- Chronic low-grade inflammation — obesity, smoking, periodontal disease (typically 3–10 mg/L range).
- Malignancy — particularly with bone marrow involvement.
What a low value can mean
- No active inflammation — the expected state in healthy adults.
- A low CRP is a strong negative predictor — useful for ruling out bacterial infection in primary care.
When to discuss with a doctor
CRP above 100 mg/L typically indicates a bacterial infection or major inflammatory process and warrants prompt evaluation. Persistent values in the 10–40 range warrant a primary-care work-up for autoimmune disease, chronic infection or malignancy. For cardiovascular risk, hsCRP >3 mg/L on two readings without acute cause is associated with elevated 10-year risk and may guide statin decisions.