A C-reactive protein (CRP) test measures levels of CRP, a protein produced in your liver, from a sample of blood. A rheumatologist (a medical doctor who specializes in musculoskeletal and inflammatory diseases) or other healthcare provider will use a CRP test to detect signs of bacterial or viral infections, diseases of the intestines or bowels, autoimmune conditions, and some lung conditions.
Let’s take a closer look at the CRP test, including what it can detect, how to get ready for your appointment, and how to interpret results.
C-reactive protein is synthesized in the liver, and typically, your levels are low. However, these levels rise in response to inflammation. Inflammation is your natural immune response to disease or infection. It increases your blood flow and helps your body cells defend against viruses, infections, or cell damage.
Since CRP tests detect CRP levels, they help to diagnose a range of conditions, including:
- Bacterial or viral infections
- Chronic inflammatory conditions, such as inflammatory bowel syndrome (IBD)
- Lupus, rheumatoid arthritis, vasculitis, and other autoimmune conditions
- Certain lung diseases, including asthma
In addition, CRP tests also play a role in seeing if treatments are working for chronic inflammatory conditions or to track the progress of sepsis, a potentially life-threatening immune response that spreads to the bloodstream. This test can also help determine your risk of heart disease, experiencing a second heart attack, or another heart condition. In addition, CRP tests can be effective for tracking the progress of a COVID-19 infection.
However, as effective as CRP tests are at detecting inflammation, they aren’t effective at detecting what exactly is causing it or where it’s located. Additional tests may be used to confirm your diagnosis.
Symptoms Prompting a CRP Test
Several symptoms, alongside other aspects of your health, may prompt your healthcare provider to order a CRP test. Signs you may need one include those of bacterial infection or other chronic conditions, such as:
- Fever or chills
- Elevated heart rate
- Rapid or shallow breathing
- Nausea and/or vomiting
- Previous positive results from testing (to track progress)
How Does a CRP Test Work?
To before CRP tests, healthcare providers, most often phlebotomists (a technician who draws blood for laboratory testing), need to collect a blood sample. These are then sent to a clinical laboratory, where pathologists (medical doctors who specialize in detecting diseases) can examine them.
Before the Test
CRP tests don’t take long, with sample collection taking less than five minutes. Though it causes a pinch, general anesthesia or numbing isn’t needed, and you won’t be hooked up to any machines. Depending on the case, you may need to provide additional medical or insurance information.
During the Test
Collecting a blood sample for testing is a rapid and relatively painless procedure. While you can expect a pinch when the needle is inserted, you shouldn’t feel excess discomfort. Here’s a quick breakdown of how it works:
- Venue: A healthcare provider will first take you to an examination room or office; typically, you are seated during the test.
- Palpation: After tying a cord around your arm, the phlebotomist or healthcare provider will first palpate—feel around—your arm to find a vein to draw from.
- Insertion: After cleaning the target area with an alcohol swab, the phlebotomist or healthcare provider inserts a needle to fill a small syringe with blood; once collected, this is sent to the laboratory.
- Cleaning: Once they collect the sample, they will remove the needle, clean and disinfect the area, and put on a bandage to stop any bleeding.
After the Test
A C-reactive protein test is an outpatient procedure, meaning you won’t need to spend a night in the hospital. Once your healthcare provider is sure there isn’t excess bleeding and that you don’t feel faint, you’ll be able to go home. There also aren’t lingering side effects, so you should be able to drive yourself and resume normal activities.
Like other blood tests, there are very few risks to the CRP test, and it’s appropriate for most to take. You may feel some soreness and discomfort around the sample collection site afterward. Some may also experience excess bleeding, bruising, or hematoma—a pooling of blood under the skin. In very rare cases, CRP tests can also cause infection.
In addition, some people have difficulty with needles and seeing blood. They may faint or have difficulty with the procedure. If this happens to you, let your healthcare provider know.
Fainting (vasovagal syncope) is caused by a sudden drop in blood pressure, which can be a reflexive response to fear. If you faint during a blood draw, you will likely be asked to have a snack and wait for a while in the exam area before leaving, just to ensure you're safe and don't faint again.
How to Prepare for the CRP Test
Since the CRP test is a type of blood test, sample collection can happen at a wide range of medical facilities, from wards in hospitals or urgent care centers to specialists’ offices. Prior to your appointment, you’ll receive more exact instructions, but here’s what to keep in mind as you prepare:
- Attire: While it’s a good idea to wear loose fitting clothing that allows easy access to your arms, there are no specific instructions for attire; unlike with certain types of imaging, you can wear jewelry.
- Food and drink: Unlike with other types of lab tests, you won’t need to fast or refrain from eating or drinking before this test.
- Medications: You may need to steer clear of non-steroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen) and magnesium supplements, which can affect your results. Prescribed medications, such as statins, can also have this effect. Your provider will let you know if you need to temporarily stop taking a prescription; don’t stop taking anything without checking with them first.
- Stop smoking: Let your provider know if you smoke; since nicotine can cause mild elevations of CRP levels, your provider may ask you to quit or refrain from smoking before the test.
- Items to bring: For any medical appointment, it’s important to have your ID, insurance information, a list of medications you’re taking, as well as any records you keep about your symptoms.
- Emotional support: If you have trouble with needles or blood draws and need a loved one or friend there with you, let your healthcare provider know.
- Cost/insurance: Since CRP tests serve an essential, diagnostic function, they’re often covered by insurance; talk to your insurance provider to understand how much, if anything, you need to cover out of pocket. Even without insurance, this test is relatively affordable, with the cost ranging from about $12 to $16.
The amount of time it takes from sample collection to results varies. Typically, it takes a day or two before your results are available. Your healthcare provider may give these over the phone, or make them accessible online in a patient web portal. If the results are abnormal or concerning, you’ll need an appointment with them to interpret the findings and discuss next steps.
Interpreting Your Results
Generally, C-reactive protein levels are low in the bloodstream, so elevated levels may indicate one of a variety of health issues. And while CRP tests are very effective in detecting inflammation in your body, additional tests may be needed to confirm the exact causes of this issue. Abnormal results prompt either more testing or the initiation of treatment.
Results of the test are typically given in terms of milligrams of protein per deciliter of blood (mg/dL). What’s considered healthy or a sign of an issue can vary based on the specific CRP test, so your provider will explain what they think your results mean.
Here’s a typical breakdown of what CRP levels can mean:
- 0.3 mg/dL and lower: This is the healthy range, showing no signs of inflammation.
- 0.3 to 1.0 mg/dL: Considered a normal or mild elevation, this range may be a result of pregnancy, diabetes, the common cold, smoking, gum disease (gingivitis), and depression, among other conditions.
- 1.0 to 10.0 mg/dL: A moderate elevation in CRP levels and inflammation in this range indicate systemic inflammation, which can be caused by a range of conditions, including rheumatoid arthritis, lupus, autoimmune diseases, cancer, heart attack, pancreatitis, or bronchitis.
- 10.0 to 50.0 mg/dL: Levels above 10, known as “marked elevation,” imply acute bacterial infections, viral infections, or systemic vasculitis (inflammation of the blood vessels).
- 50.0 mg/dL or higher: Severely elevated levels of CRP are a sign of bacterial infection about 90% of the time.
A Quick Review
C-reactive protein (CRP) is produced in the liver and found in the blood. While CRP levels are usually very low, elevated levels are a sign of inflammation, one of your body’s natural immune responses. CRP tests are used to detect signs of a bacterial or viral infection, arthritis, autoimmune diseases, or cancer, or track to the progress of treatment.
A CRP test is quick procedure consisting of a drawing a sample of blood from your arm. The sample then goes off to a clinical laboratory for evaluation. While CRP tests are sensitive to signs of infection or inflammation, other tests are usually needed to confirm a diagnosis.
