C-reactive protein is a substance that the liver makes in response to inflammation. The C-reactive protein test measures the amount of this protein in the blood. The test can help to diagnose acute and chronic conditions that cause inflammation.
A wide variety of inflammatory conditions can cause elevated C-reactive protein (CRP) levels, including the following:
- autoimmune conditions, including rheumatoid arthritis (RA), lupus, and certain types of inflammatory bowel disease, such as Crohn's disease and ulcerative colitis
- pericarditis, which is inflammation of the lining of the heart
- organ and tissue injury
Significantly high CRP levels of more than 350 milligrams per liter (mg/L) are nearly always a sign of a serious underlying medical condition. The most common cause is a severe infection, but a poorly controlled autoimmune disease or severe tissue damage can also lead to high CRP levels.
There is no need to fast or avoid liquids before having a CRP test. However, people having a high-sensitivity CRP (hs-CRP) test are likely to undergo other blood tests at the same time, and these may require fasting for 9–12 hours beforehand.
The hs-CRP test is different from the CRP test. The hs-CRP test detects lower levels of CRP in the bloodstream (0.5–10 mg/L), whereas the CRP test measures levels in a higher range (10–1,000 mg/L). Doctors will use the hs-CRP test to evaluate a person's risk of developing cardiovascular disease.
Some conditions that may cause an elevated hs-CRP level include:
Obtaining the sample for the test will only take a few minutes and should be relatively painless aside from a small needle prick.
The symptoms of elevated CRP levels depend entirely on the underlying condition that is causing them.
Many people who have moderate infections or injuries, or conditions that cause chronic inflammation, may experience similar symptoms. These include:
- unexplained exhaustion
- muscle stiffness, soreness, and weakness
- low-grade fever
- a headache
- nausea, loss of appetite, and indigestion
- difficulty sleeping or insomnia
- unexplained weight loss
People with very high CRP levels are most likely to have an acute bacterial infection.
Signs of acute infection include:
There is currently no set standard for CRP blood levels, and guidelines vary.
However, as a general rule, the following classifications apply to CRP:
- Levels between 3 mg/L and 10 mg/L are mildly elevated and usually result from chronic conditions such as diabetes, hypertension, or lifestyle factors including tobacco smoking and being sedentary.
- Levels between 10 mg/L and 100 mg/L are moderately elevated and are usually due to more significant inflammation from an infectious or non-infectious cause.
- Levels above 100 mg/L are severely elevated and almost always a sign of severe bacterial infection.
The hs-CRP test results indicate a person's risk of developing cardiovascular disease accordingly:
- Low risk is less than 1 mg/L.
- Moderate risk is between 1 mg/L and 3 mg/L.
- High risk is greater than 3 mg/L.
CRP levels that are mildly or moderately elevated can be hard to interpret.
A huge range of conditions can raise CRP levels slightly, and, as there is no standard reference range set for CRP, there is usually no way to draw any conclusions by looking at CRP levels alone.
The following factors can also make it challenging to interpret CRP levels:
- Medications: Medications that reduce inflammation in the body, such as some cholesterol-reducing medicines (statins) and specific nonsteroidal anti-inflammatory drugs (NSAIDs), may lower CRP levels.
- Minor injuries or infection: These conditions may temporarily raise CRP levels and conceal other potential conditions such as diabetes or IBS.
- Chronic conditions: Conditions that cause persistent inflammation, including autoimmune diseases, may mask other possible causes of elevated CRP such as minor infection.
- Estrogen levels: Estrogen-based medications such as birth control pills and hormone replacement medications may raise CRP levels.
- Pregnancy: Pregnancy can elevate CRP levels, especially during the later stages.
A doctor will usually order a CRP test alongside several other tests to get a broad overview of a person's health. This will allow them to consider a range of medical factors.
A doctor will also often want to repeat the test to see how CRP levels change over time before they make a diagnosis.
However, regardless of any other external factors, CRP levels above 10 mg/L will typically indicate an underlying inflammatory issue.
Usually, moderately elevated CRP levels are due to RA or infectious arthritis, which occurs when a joint is infected.
Significantly elevated CRP levels tend to occur with severe infections, such as bacterial or fungal infections. Infection is responsible for around 80 percent of the cases involving CRP levels greater than 10 mg/L.
Sometimes higher levels also occur due to certain cancers and other conditions that can cause significant inflammation, such as pericarditis.
Doctors use CRP and hs-CRP tests to detect the levels of CRP in the body. Elevated levels of the protein may indicate an underlying condition or a higher risk of cardiovascular disease. The treatment for elevated levels will differ according to the cause.
A range of conditions can cause mildly or moderately raised CRP levels, but very high CRP levels are generally easier to interpret.