What the CRP test shows
C-reactive protein (CRP) is a protein the liver releases within hours of inflammation, infection or tissue injury. It is the classic acute-phase reactant: levels can climb more than a thousand-fold and, with a half-life of about a day, fall just as fast once the trigger clears. As MedlinePlus puts it, the test measures inflammation.
CRP is very sensitive but not specific: a high value proves inflammation is present, not where or why. That directness sets it apart from the ESR, another general inflammation marker — both track the acute-phase response, but CRP moves within hours while ESR lags over days. When more detail is needed, procalcitonin is more selective for bacterial infection, and interleukin-6 is the cytokine that tells the liver to make CRP.
The same protein is measured two ways: a standard CRP when you feel unwell, and a more sensitive hs-CRP that reads the low end to gauge cardiovascular risk in people without symptoms.
CRP normal range
CRP is reported in mg/L (its SI unit is also mg/L), but some US labs use mg/dL — and since 1 mg/dL equals 10 mg/L, the same result can look ten times bigger or smaller, so check your report’s unit. CRP has no strong sex- or age-specific range like ferritin; its baseline only drifts up with older age, higher body weight, smoking and late pregnancy, so it is read by level band.
| CRP, mg/L (mg/dL) | What it usually reflects |
|---|---|
| Under 3 (under 0.3) | Normal, low-grade |
| 3–10 (0.3–1.0) | Minor rise — obesity, smoking, late pregnancy, chronic low-grade inflammation |
| 10–100 (1.0–10) | Moderate — active inflammation, autoimmune flare, many infections, heart attack |
| Over 100 (over 10) | Marked — serious bacterial infection, systemic vasculitis, major trauma |
For cardiovascular screening, the more sensitive hs-CRP uses tighter bands from the AHA and CDC:
| hs-CRP, mg/L | Cardiovascular risk |
|---|---|
| Under 1.0 | Low |
| 1.0–3.0 | Average |
| Over 3.0 | High |
An hs-CRP above 10 mg/L is not read as heart risk — it reflects infection or other inflammation and should be repeated once you are well. Ranges depend on the lab, method, sex and age, so read your result against your own report.
Why CRP is high
A high CRP points to inflammation but not its source. Roughly by frequency:
- Infection — the commonest reason. Bacterial infections drive the biggest rises, often above 100 mg/L; viral ones are usually milder. Values in the hundreds — above about 500 mg/L (50 mg/dL) — mean serious bacterial infection roughly 90% of the time, per StatPearls.
- Tissue injury: surgery, trauma, burns or a recent heart attack; after surgery CRP peaks around day two or three, and a rising value can signal a complication.
- Autoimmune disease: rheumatoid arthritis, lupus, vasculitis, polymyalgia rheumatica and inflammatory bowel disease, which CRP can track.
- Chronic low-grade inflammation: obesity, metabolic syndrome, type 2 diabetes and smoking, which keep hs-CRP raised.
- Cancers, particularly lymphoma and some solid tumors.
When is it urgent? A very high CRP — often above 100 mg/L, especially if rising — with fever, chills, severe pain, breathlessness or confusion can mean serious bacterial infection or sepsis, which needs same-day care.
Why CRP is low
A low CRP is normal and desirable, and an hs-CRP under 1 mg/L is simply the low-risk band. The real caution runs the other way: a normal CRP does not rule illness out.
- Timing: CRP takes 6–12 hours to rise, so it can read normal very early in an infection; a repeat a day later may differ.
- Localized infection: an early or walled-off abscess can raise CRP only modestly.
- A weak acute-phase response: because the liver makes CRP, advanced liver failure can blunt the rise — the Mayo Clinic stresses reading it in context.
- Medicines: statins lower CRP a little, and interleukin-6 blockers such as tocilizumab or sarilumab can flatten it almost completely, so a normal CRP cannot rule out infection in those patients.
So a normal CRP is always read with symptoms, and clinicians may turn to procalcitonin or a repeat test.
What to test alongside
CRP is rarely read alone. It is paired with markers that confirm inflammation and point toward its cause:
- ESR — the slower, complementary inflammation marker.
- Procalcitonin — more specific for bacterial infection and sepsis.
- Interleukin-6 — the cytokine that drives CRP.
- Rheumatoid factor — for a persistent, unexplained rise.
- Anti-CCP — a more specific antibody for rheumatoid arthritis.
- ASO — evidence of a recent streptococcal infection.
- Ferritin — another acute-phase protein; anemia of chronic disease.
- Hemoglobin — chronic inflammation often causes a mild anemia.
- LDL cholesterol — paired with hs-CRP for cardiovascular risk.
- Creatinine — a kidney check; kidney disease raises CRP.
What to do about an abnormal result
- Don’t treat a number. CRP is a clue, not a diagnosis; a mild, isolated rise with no symptoms often needs only a repeat.
- Read it in context — with your symptoms, the ESR and the blood count — repeating a borderline value once you are well.
- Act on danger signs. A high CRP with fever, rigors, severe or spreading pain, breathlessness or confusion needs urgent, same-day assessment, because sepsis is time-critical.
- See your primary-care doctor first for a persistent, unexplained rise; they look for hidden infection, autoimmune or kidney disease and refer on when needed.
- For a raised hs-CRP, weigh it with your lipids, blood pressure and HbA1c; stopping smoking, weight loss, exercise and, where indicated, a statin lower it. It helps to read the report against your lab’s ranges first.
Mini-FAQ
What is a normal CRP level?
In healthy adults CRP is usually under about 3 mg/L (0.3 mg/dL), and a standard test is often only flagged above 5–10 mg/L. Watch the units (1 mg/dL = 10 mg/L) and read against your own lab’s range.
What does a high CRP mean?
A raised CRP signals inflammation somewhere but not where or why. Infection is the commonest cause, then tissue injury, autoimmune disease and chronic low-grade inflammation, so it is read with your symptoms, ESR and other tests.
What is hs-CRP, and how is it different from a regular CRP?
It is the same protein measured with a more sensitive assay that reads the low end. hs-CRP estimates cardiovascular risk: under 1 mg/L is low, 1–3 mg/L average and over 3 mg/L high. Over 10 mg/L points to infection, not heart risk, and should be rechecked when well.
How is CRP different from ESR?
CRP is measured directly and rises and falls within hours; ESR is an indirect measure that changes over days. CRP is usually better for tracking acute illness, while ESR keeps a role in conditions such as giant cell arteritis and polymyalgia.
When is a high CRP a medical emergency?
A very high CRP — often above 100 mg/L — with fever, shaking chills, severe pain, breathlessness or confusion can point to serious bacterial infection or sepsis and needs same-day or emergency care. It is always read with how unwell you are.
