What Does a High CRP Mean?
A doctor explains what C-reactive protein measures, what different CRP levels suggest, and why a raised CRP is a signpost rather than a diagnosis.
The short version
- CRP is a protein the liver makes when there is inflammation anywhere in the body; a high result tells you inflammation exists but not where or why.
- Most laboratories treat under 5 mg/L as normal; levels above about 100 mg/L usually point to significant bacterial infection, while mild elevations have many causes.
- A single raised CRP in someone who feels well is often a recent infection, injury or obesity-related inflammation, and repeating it after a few weeks is usually the sensible next step.
See a doctor promptly if
These are the signs that change this from something to read about into something to act on.
- A high CRP with fever, confusion, a very fast heart rate or breathing rate - possible sepsis, seek emergency care
- A high CRP with severe abdominal pain, or with a hot swollen joint
- A persistently raised CRP over weeks with weight loss or night sweats
CRP, or C-reactive protein, is a protein your liver releases into the blood when there is inflammation somewhere in the body. A high CRP therefore tells you that inflammation is present. It does not tell you where the inflammation is, or what is causing it, which is why it is a signpost rather than a diagnosis.
It is a genuinely useful test for exactly two things: judging how much inflammation there is, and tracking whether it is rising or falling over time.
How to read the number#
| CRP (mg/L) | What it usually suggests |
|---|---|
| Under 5 | Normal in most laboratories |
| 5 to 40 | Mild inflammation - viral illness, recent injury, chronic conditions |
| 40 to 100 | Moderate - significant infection or active inflammatory disease |
| Over 100 | Usually significant bacterial infection, sometimes major inflammation |
These bands overlap considerably and no number should be read in isolation. A CRP of 60 in someone with a two-day fever and a cough means something quite different from a CRP of 60 in someone who feels entirely well.
What raises it#
Bacterial infections raise CRP the most, typically far higher than viral infections. Surgery and injury raise it for several days. Autoimmune and inflammatory conditions such as rheumatoid arthritis and inflammatory bowel disease raise it during flares, which is why it is used to monitor them.
Less dramatic influences matter too: obesity, smoking, poorly controlled diabetes and pregnancy all nudge CRP upward. Vigorous exercise can raise it briefly. This is why a mild elevation in someone who feels perfectly well is common and usually unimportant.
Why the trend matters more than the value#
CRP rises within about six to twelve hours of an inflammatory insult and peaks at roughly forty-eight hours. It also falls quickly, with a half-life of about nineteen hours, once the cause resolves. That fast response makes serial measurements informative: a CRP falling from 150 to 40 over three days tells a doctor that treatment is working, which a single reading never could.
It also means a normal CRP very early in an illness does not exclude infection.
What a doctor does with a high result#
They interpret it against how you actually are. A raised CRP with fever, a fast heart rate, fast breathing or confusion raises concern for serious infection and is treated urgently. A raised CRP with a hot swollen joint, or severe abdominal pain, prompts a specific search. A raised CRP with no symptoms usually prompts a repeat, a full blood count, and a look for the quiet causes.
CRP is often ordered together with ESR, an older inflammation marker that changes more slowly. The two are complementary rather than duplicates.
The wider picture#
CRP rarely arrives alone on a report. The fuller guide to understanding your blood test results explains how it sits alongside the full blood count, liver and kidney tests, and what a pattern across them means.
Common questions
What is a normal CRP?
Is high-sensitivity CRP the same test?
Can a normal CRP rule out infection?
Sources
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