Explainer · updated 2026-09-09
C-reactive protein, explained in plain English
Key facts
- Upper end of the typical adult reference interval: 5 mg/L (HRI-HR)
- The same upper end in US units (converted from 5 mg/L): 0.5 mg/dL (HRI-HR)
- Level above which a reading counts toward inflammatory-illness diagnoses such as vasculitis: about 10 mg/L (NICE 2023)
- What CRP is: a protein the liver pours out within hours of inflammation starting anywhere in the body (NICE 2023)
- What a low level means: no significant inflammation right now, which is the normal finding (NICE 2023)
What it is
C-reactive protein, usually written CRP, is a protein the liver pours out within hours of inflammation starting anywhere in the body. It rises and falls far faster than the sedimentation rate, which is what makes it useful for tracking a problem day to day. It is measured in the blood.
What the number is telling you
The level reflects whether inflammation is active right now. Because CRP moves quickly, the same test can be low one week and raised the next, and that speed is the point: it lets a doctor follow a problem day to day rather than over weeks. One reading says little; time and trend matter.
What high or low can mean
High: A high reading means inflammation is active somewhere in the body, whether from infection, injury or an inflammatory illness. Above about 10 mg/L a reading counts toward inflammatory-illness diagnoses such as vasculitis, and below 20 mg/L it argues against a bacterial infection needing antibiotics. Where your own level sits is for your doctor to say.
Low: A low level means no significant inflammation right now, the normal finding. The high-sensitivity version of the test reads very low levels against heart-risk bands, which is a separate scale.
What a typical result looks like
Typical adult reference intervals from the HRI-HR harmonized set, verified in Croatia and published in Biochemia Medica in 2026, put the upper end of C-reactive protein at 5 mg/L, about 0.5 mg/dL. The range on your own report is the one that counts.
Questions worth asking your doctor
- Was I unwell or inflamed when this sample was taken?
- How does this compare with my previous results?
- If my level is raised, what are we looking for, and how will we follow it?
- How often does this test need repeating for me?
If a result worries you, ask your doctor.
Sources
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