C-reactive protein · hs-CRP · inflammation marker
CRP: what an inflammation marker means
CRP rises when something in the body is inflamed. It is one of the most useful tests in medicine and one of the least specific — it says how much, never where.
What it measures
C-reactive protein is made by the liver in response to inflammation. It rises within hours, peaks in a day or two, and falls quickly once the cause resolves, which makes it good at showing whether something is getting better or worse. A high-sensitivity version (hs-CRP) resolves the low end of the range and is used for cardiovascular risk rather than for infection.
Why it gets tested
To help tell bacterial illness from viral, to follow an infection or a flare of an inflammatory condition, after surgery, and to monitor treatment in rheumatoid arthritis or inflammatory bowel disease. hs-CRP is sometimes added to cardiovascular risk assessment when the decision is borderline.
Typical ranges
Most laboratories treat below 5 mg/L as normal. Mild elevations of 5–10 mg/L are common and often unexplained; 10–100 mg/L suggests significant inflammation; above 100 mg/L usually means bacterial infection or major tissue injury. For hs-CRP in cardiovascular risk, below 1 mg/L is low, 1–3 average, and above 3 higher.
Compare against the range printed on your own report, not this one. Laboratories differ, and yours is the one your doctor is reading.
What a high result can mean
Infection is the commonest cause, but any inflammation will do it — an autoimmune flare, injury, surgery, a burn, pancreatitis, and some cancers. Persistent low-grade elevation accompanies obesity, smoking, gum disease and metabolic syndrome. CRP cannot tell these apart: it reports the fire alarm, never the room.
What a low result can mean
A low or undetectable CRP is normal and reassuring, and needs no action. It does not rule everything out — some conditions, lupus notably, can flare with a CRP that stays low — so it is read alongside symptoms rather than instead of them.
What moves it
Any infection or injury in the preceding days, surgery, strenuous exercise, pregnancy, obesity and smoking. Statins and anti-inflammatory treatment lower it. Because it moves so fast, when the sample was taken relative to an illness matters more for CRP than for almost any other routine test.
What usually happens next
What follows depends entirely on the clinical picture, because CRP localises nothing. A high result in someone unwell prompts a search for the source. A mildly raised one in someone well is usually just repeated after a few weeks, since transient elevation after a minor illness is extremely common — but a persistently raised CRP with no explanation does get investigated.
Common questions
Does a high CRP mean I have an infection?
Not necessarily. It means inflammation, and infection is only one cause among injury, autoimmune disease and recent surgery. How high it is helps — very high levels point towards bacterial infection — but the test can never say where the inflammation is.
What is the difference between CRP and hs-CRP?
The same protein, measured with different precision. Standard CRP detects and follows inflammation; high-sensitivity CRP resolves the low end of the range and is used to refine cardiovascular risk. Ordering hs-CRP during an infection is not informative — anything acute swamps the signal it is looking for.