What this test measures
C-Reactive Protein (CRP) is an acute-phase protein made by the liver in response to inflammation anywhere in the body — bacterial infection, viral infection (less so), tissue injury, autoimmune flare, or post-operative recovery. CRP rises within 6–8 hours of the trigger, peaks at 24–48 hours, and falls quickly once inflammation resolves — making it a sensitive real-time tracker.
The "standard" CRP measured here is in milligrams per litre (mg/L) and is sensitive down to about 1 mg/L. A separate test, high-sensitivity CRP (hs-CRP), measures down to 0.1 mg/L and is used to assess long-term cardiovascular risk in seemingly healthy adults — see the hs-CRP page for that use case.
Why it matters
CRP is one of the cheapest and fastest ways to confirm that the body is fighting something. It is used to: distinguish bacterial vs viral infections (very high CRP favours bacterial); monitor treatment response (a falling CRP means therapy is working); track flares in autoimmune disease (rheumatoid arthritis, lupus, IBD); check for post-operative infection; and as part of a sepsis workup. CRP is non-specific — it does not tell you where the inflammation is — but combined with clinical context it is genuinely useful.
Note that older / frail patients with serious infection can sometimes have a blunted CRP rise, and obesity can chronically elevate CRP by 2–5 mg/L without acute illness. Interpretation always belongs with a doctor.
How to prepare
No fasting required. The test can be done at any time. Continue your usual medications. NSAIDs (ibuprofen, diclofenac) and steroids can lower CRP and may mask the true inflammation level — tell your doctor what you have taken. CRP can rise transiently after vigorous exercise; avoid heavy exercise on the test day if possible.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| C-Reactive Protein (CRP) (mg/L)[1][2][3] | < 10 (Indian lab standard) · < 5 (more sensitive cut-off used by some labs) | Low or undetectable CRP is normal — it means there is no active inflammation. CRP is essentially undetectable in healthy people; the lower limit of "normal" is set by assay sensitivity rather than physiology. | 10–40 mg/L: mild to moderate inflammation — viral infection, mild bacterial infection, autoimmune flare, post-operative day 1–3. 40–200 mg/L: moderate to marked inflammation — bacterial infection (UTI, pneumonia, cellulitis), active rheumatoid arthritis, significant autoimmune flare. > 200 mg/L: severe inflammation — sepsis, severe bacterial infection, large abscess, severe autoimmune disease, post-major-surgery. |
How to read your CRP result
| CRP (mg/L) | Level | What it usually means |
|---|---|---|
| < 10 | Normal | No significant active inflammation |
| 10 – 40 | Mild / moderate | Viral infection, mild bacterial infection, post-operative day 1–3, autoimmune flare |
| 40 – 100 | Moderate | Bacterial infection (UTI, pneumonia, cellulitis), active inflammatory arthritis |
| 100 – 200 | Marked | Significant bacterial infection, abscess, IBD flare, post-major-surgery |
| > 200 | Severe | Sepsis, severe bacterial infection, severe autoimmune disease, major tissue injury |
Frequently asked questions
Do I need to fast for a CRP test?
No. CRP can be tested at any time of day with no fasting required.
What is the difference between CRP and hs-CRP?
They measure the same molecule but with different assay sensitivities. Standard CRP (down to ~1 mg/L) is used to assess active infection or inflammation. High-sensitivity CRP (hs-CRP, down to 0.1 mg/L) is used to estimate cardiovascular risk in apparently healthy adults — it cannot replace standard CRP in the setting of suspected acute infection.
My CRP is 30 — what does that mean?
A CRP of 30 mg/L is mild to moderate inflammation. The likely causes are a mild bacterial infection, a viral infection winding down, a flare of an autoimmune condition, or recovery from minor surgery / injury. CRP cannot tell you the source — that needs the clinical context and possibly other tests. See a doctor if you have symptoms.
Does CRP rise with viral infections?
Mildly. Viral infections typically push CRP up to 10–40 mg/L. Higher levels (>100) are more suggestive of a bacterial cause. This is one of the reasons CRP is sometimes used to support a decision about whether to prescribe antibiotics for a respiratory infection.
How quickly does CRP change?
CRP starts rising within 6 hours of the inflammatory trigger, peaks at 24–48 hours, and falls by about 50% per day once inflammation is treated. That makes serial CRP measurements useful to track treatment response.
Can painkillers affect my CRP?
Yes. NSAIDs (ibuprofen, diclofenac) and steroids (prednisolone) reduce inflammation and can lower CRP — sometimes masking serious infection in the early stages. Always tell your doctor what medications you are on.
Why is my CRP slightly raised when I feel fine?
Persistent low-grade CRP rise (10–20 mg/L) can occur with obesity, diabetes, smoking, periodontitis (gum disease), undiagnosed autoimmune disease, or chronic infections. If unexplained, your doctor may order additional tests to look for a cause.
How is CRP different from ESR?
Both are inflammation markers but they behave differently. CRP rises and falls quickly (hours to days). ESR (erythrocyte sedimentation rate) rises and falls slowly (days to weeks) and is influenced by age, anemia, and pregnancy. CRP is more useful for acute changes; ESR is sometimes preferred for chronic inflammatory disease monitoring. Doctors often order both together.
Related Cardiac Markers tests
Tests commonly ordered alongside C-REACTIVE PROTEIN (CRP), or that help interpret an unexpected result.
Sources & references
- AHA Scientific Statement on CRP and Cardiovascular Risk · accessed 2026-05-29T00:00:00.000Z
- NIH MedlinePlus — C-Reactive Protein Test · accessed 2026-05-29T00:00:00.000Z
- NCBI StatPearls — C Reactive Protein · accessed 2026-05-29T00:00:00.000Z
- NICE — Inflammatory Markers in Primary Care · accessed 2026-05-29T00:00:00.000Z
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