CRP
C-Reactive Protein
What this test measures
CRP is a protein made by your liver when there is inflammation in your body. This test can help show if you have an infection or an inflammatory condition. CRP rises quickly when something is wrong.
Understanding the numbers
Typical range: Below 5 mg/L
Levels above 10 mg/L usually suggest significant inflammation or infection.
- Normal: 0–5 mg/L
- Mild inflammation: 5–40 mg/L
- Moderate inflammation: 40–100 mg/L
- Severe inflammation / serious infection: 100–400 mg/L
Key numbers
- 5 mg/L — Upper limit of normal
- 40 mg/L — Above = more than a mild cold / viral URTI
- 100 mg/L — Strongly suggests bacterial infection / serious inflammation
- 20 mg/L — NICE pneumonia: consider antibiotics around this level
NICE CG191 pneumonia: CRP <20 usually no antibiotics, 20-100 delayed / patient preference, >100 antibiotics indicated (in primary care). CRP is faster than ESR — rises within hours of inflammation and falls within days of resolution, so useful for tracking response to treatment.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your body is fighting inflammation somewhere
- Common causes include infections, injuries, or inflammatory conditions
- Conditions like rheumatoid arthritis can raise CRP
- Your GP will look at your symptoms to find the cause
Questions you could ask your GP
- What might be causing the inflammation in my body?
- Do I need further tests to find the source of the inflammation?
- Should I have a repeat CRP test to see if it is improving?
- Could this be related to an ongoing condition like arthritis?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Viral URTI — low CRP
Niamh had a cough and sore throat for four days. CRP was 18. No focal signs of pneumonia.
What the GP said: “Your CRP is only slightly raised — this is more consistent with a viral infection than a bacterial one. Self-care, paracetamol, fluids. Come back if you get worse or the cough lasts more than three weeks.”
What happened next: Niamh recovered within a week without antibiotics.
Mildly raised CRP often sits with viral infection. Starting antibiotics at this level usually doesn't help — and NICE uses CRP <20 in primary care pneumonia guidance to support not prescribing.
Severe pneumonia
Bruce had a productive cough and fever. CRP 210, oxygen 91%, CURB-65 high.
What the GP said: “A very high CRP with fever and low oxygen strongly suggests bacterial pneumonia needing admission. I'm sending you to hospital for IV antibiotics and oxygen.”
What happened next: Admitted four days; IV co-amoxiclav then oral step-down; CRP fell from 210 to 18 over a week.
A CRP over 100 with clear clinical features supports serious bacterial infection and usually antibiotic treatment. CRP is also useful for monitoring — a falling CRP on treatment is reassuring.
Active rheumatoid arthritis
Suresh has rheumatoid arthritis. CRP 65 and ESR 62 with active joint swelling. No infection.
What the GP said: “Your rheumatoid is active. I'll liaise with your rheumatology team about stepping up treatment. We use CRP to track how well disease-modifying drugs are working.”
What happened next: After a dose adjustment of adalimumab, CRP came down to 6 over two months.
CRP and ESR are used to track inflammatory disease activity (rheumatoid, polymyalgia, vasculitis). Treatment success is measured partly by bringing inflammation back to baseline.
Polymyalgia rheumatica
Jane had three weeks of shoulder and hip girdle stiffness worse in the morning. CRP 95, ESR 85.
What the GP said: “This looks like polymyalgia rheumatica — a classic pattern in older adults. A short course of prednisolone usually gives a dramatic response within days; that response also confirms the diagnosis.”
What happened next: Within 48 hours on 15 mg prednisolone, Jane's stiffness was gone. CRP fell to 8 within two weeks.
In polymyalgia, a markedly raised CRP with the right symptoms and a rapid steroid response is the diagnosis. Always ask about temporal-artery symptoms (jaw claudication, scalp tenderness, visual changes) — giant cell arteritis can coexist and needs higher-dose steroids quickly.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.