A CRP blood test measures C-reactive protein, which your liver makes when your body is inflamed. Levels rise sharply with infection, injury and many illnesses, so doctors use it to spot and track inflammation. It is a good alarm but a poor map: it can't say what is inflamed, so it is read with your symptoms and exam.
What does CRP stand for?
CRP is short for C-reactive protein. The name is a leftover from its discovery. In 1930, scientists found it in the blood of people in the early, acute phase of pneumococcal pneumonia. They named it for the way it stuck to a sugar in the bacteria's outer wall, called the C-polysaccharide.
Later work filled in the rest. The protein is a ring of five identical pieces. It is made in liver cells. And it is there in healthy blood too, just in trace amounts.
The liver turns up production when the immune system sends out alarm signals. Those signals include IL-6, short for interleukin 6, along with IL-1 and TNF. When you are inflamed, the level of CRP in your blood climbs sharply. That is why CRP is called an "acute phase" protein: part of the body's fast response to injury and infection.
What does a CRP blood test measure?
The test is a standard blood draw. The lab measures how much CRP is in your blood.
CRP says something is inflamed. It does not say what.From a 2016 review in WMJ
CRP was the first protein of its kind used in clinical labs. It is a very sensitive marker of inflammation and tissue damage across the whole body, and the level helps doctors judge how severe the inflammation is.
There are two versions. The standard CRP test is built to catch the big rises that come with infection or injury. Newer high-sensitivity tests, called hs-CRP, can read much lower levels. That lets doctors look for the quiet, low-grade inflammation that builds up behind blocked arteries. Same protein, different job.
Why would a doctor order a CRP test?
Mostly to answer one of three questions.
Is this an infection, and how serious is it? CRP is widely used to diagnose and monitor inflammatory conditions, including sepsis, injuries and cancers. It is one of the most requested lab tests there is.
Do I need antibiotics? Most chest colds and other airway infections come from viruses or mild bacteria, so antibiotics often do not help. A 2022 Cochrane review pooled 12 randomized trials with 10,218 people seen in primary care. When doctors used a quick CRP test right in the clinic, 397 of every 1,000 people left with an antibiotic prescription, compared with 516 of every 1,000 with usual care. Recovery within a week was about the same.
What is my heart risk? This is where hs-CRP comes in. A 2010 analysis pooled 160,309 people without heart disease from 54 long-term studies. Each threefold step up in CRP was linked to about 37% more coronary heart disease, after accounting for the usual risk factors. A 2025 American College of Cardiology statement says that in people who seem healthy, a high hs-CRP marks a higher-risk group, even when LDL cholesterol is normal.
Is CRP a good test for inflammation?
It is good at noticing. It is poor at explaining. Even among doctors, the best way to use it is still debated.
A 2016 review put it plainly: CRP and the sed rate (ESR), its older cousin, lack sensitivity and specificity. They should be used only alongside a person's history and a physical exam. They work best when a doctor already thinks a disease is either quite likely or quite unlikely.
The numbers show why. In a 2020 meta-analysis of 29 studies, CRP picked up 86 of every 100 people who had an inflammatory condition. But it also flagged about 1 in 3 people who did not. For sepsis, a dangerous whole-body reaction to infection, a 2025 analysis of 44 studies found CRP's power to tell sick from not-sick was limited.
The big heart study had its own twist. Higher CRP was also linked to deaths from several cancers and lung disease, and the authors said the meaning of CRP across such a range of illnesses was unclear. A high CRP says something is inflamed. It does not say what.
Then there is the wobble. A 2024 review of 60 studies looked at people who had CRP measured more than once. The typical swing from test to test was about 41% for standard CRP and 44% for hs-CRP. The authors called this variation high, though many of the studies were small.
Weight shows up here too. In a 2021 study of 1,472 Spanish adults with no known inflammatory illness, 208 had a high CRP with a normal sed rate, and that pattern was linked to a higher body mass index. BMI was the main thing tied to CRP levels in that group.
What this means for you
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you have a high CRP along with fever, new pain, weight loss you can't explain or night sweats; you live with an inflammatory condition such as arthritis or bowel disease and your result changed; you are pregnant; or you take medicines that affect inflammation or cholesterol. Keep taking any medicine as prescribed, and ask before changing it based on a CRP result.
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The bottom line
A protein first spotted in pneumonia patients nearly a century ago is now one of the most ordered tests in medicine. It earns that place. It climbs sharply when you are inflamed, results come back quickly, and in the clinic it has helped doctors give fewer antibiotics without slower recovery. But the same evidence draws its limits: it can't name the source, it flags some healthy people, and it moves around in the same person from test to test. Treat your CRP as a smoke alarm. It tells you to look. Your doctor, your symptoms and sometimes a second test tell you where.
Common questions
What does CRP stand for?
CRP stands for C-reactive protein. It was discovered in 1930 in the blood of people with pneumococcal pneumonia and was named for the way it binds to the C-polysaccharide in the bacteria's cell wall. It is made in the liver, and its level rises sharply when the body is inflamed.
Why would a doctor order a CRP test?
Usually to check for infection or to track an inflammatory illness. CRP is used to diagnose and monitor conditions such as sepsis, injuries and some cancers. In primary care, a quick CRP test can help decide whether a chest infection needs antibiotics. A high-sensitivity version, hs-CRP, is used as part of heart risk checks.
Is CRP a good test for inflammation?
It is a sensitive sign that inflammation is present, but not a specific one. A 2016 review says CRP should only be read with a person's history and exam. In a meta-analysis of 29 studies, it caught 86 of 100 people with an inflammatory condition but also flagged about 1 in 3 without one. Levels also swing between tests.
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How this article was made
Researched and written from 12 peer-reviewed papers. Our automated check matched every claim against the exact words of its source, confirmed every paper exists in PubMed, and found none retracted. Checked November 2, 2026. Not individually reviewed by a doctor. Photos are illustrative; the people in them are not patients or readers.
Grades run Strong, Moderate, Emerging, Early and Not supported, and the grade decides the words we may use. How we grade evidence · Medical disclaimer
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