What Is a C-Reactive Protein (CRP) Test?

The Test That Can Detect Inflammation

vials of blood
Tetra Images/Getty Images

The C-reactive protein (CRP) test measures the concentration of CRP, a protein that's produced in your liver, in your blood. During episodes of acute inflammation or infection, your CRP levels increase as the protein interacts with your body's complement system, a part of your immune system's defense mechanism that helps eliminate pathogens such as bacteria and viruses.

Purpose of Test

You may have a CRP test to check for inflammation in your body due to an infection or a chronic inflammatory disease or to assess your risk of developing heart disease. Though this test can detect inflammation, it doesn't show where the inflammation occurs or what's causing it. Because of this, it's considered a general indicator, not a specific test.

Another blood test that's often ordered together with a CRP test is known as the erythrocyte sedimentation rate (ESR or sed rate), which also looks for inflammation. Both CRP and ESR yield non-specific information about inflammation, but one notable difference between the two tests is that changes are reflected more quickly with CRP compared to ESR. For example, your CRP level may drop to normal following successful treatment of an infection more quickly, while ESR remains elevated for a longer period.

Infection

You may have a CRP test if your doctor suspects you have a fungal infection or a severe bacterial infection like tuberculosis, sepsis, or pneumonia. Again, the test won't show where the infection is or what's causing it, but if your CRP level is high, this tells your doctor that his or her suspicion of a severe infection is likely correct and that more testing may be necessary to find the source. You may also have a CRP test done when you've finished treatment for an infection to make sure that inflammation isn't still present in your body and the treatment has been successful.

Chronic Inflammatory Disease

In cases of chronic inflammatory diseases, such as rheumatoid arthritis (RA), inflammatory bowel disease (IBD), and lupus, your doctor can utilize the CRP test to assess how effective a specific treatment is and monitor any periods of disease flare-up. With inflammatory disease, a low CRP level is possible but doesn't necessarily indicate that there is no inflammation present. CRP levels may not be increased in some people diagnosed with rheumatoid arthritis or lupus, but the reason for this is unclear.

This test may also be used if one of these or another inflammatory disease is suspected, perhaps because you're presenting with symptoms such as fatigue, fever, and weight loss. While results alone cannot determine a diagnosis, they can be a piece of the puzzle that helps your doctor get to the bottom of why you feel the way you do.

In cases where rheumatoid arthritis is suspected in particular, your doctor may order additional blood tests that look at your rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) antibodies. In people with RA, these antibodies are often elevated. If your doctor suspects lupus, she may order other blood tests as well such as an antinuclear antibody (ANA) test and a kidney and liver function test.

Heart Disease Risk

There is a high-sensitivity CRP test (hs-CRP) in addition to the regular CRP test. The hs-CRP measures very low amounts of CRP in your blood and is typically used to assess your risk for developing coronary artery disease, a condition that's caused by atherosclerosis (hardening of the arteries). Your doctor will likely order a cholesterol test along with a CRP test since the same blood sample can be used and they can both assess heart disease risk.

Studies have indicated that having a high CRP level may be linked to an increased risk of heart attack. In fact, considering that only around 50 percent of people who have a heart attack have high LDL cholesterol (the "bad" cholesterol), the CRP test can catch heart attack risk in people who have normal cholesterol and wouldn't otherwise have been flagged.

That said, the American Heart Association isn't recommending that everyone gets an hs-CRP test; it's most helpful for people who have an intermediate risk of heart disease, which is defined as a 5 percent to 10 percent chance of having a heart attack within the next 10 years. This risk is calculated by factoring in your family history, current health conditions, and your lifestyle habits.

Having an elevated CRP level doesn't necessarily mean that your risk of heart disease is higher, however. Remember, this test doesn't show where the inflammation is, just that there is inflammation somewhere in your body.

Your doctor may also use the hs-CRP to monitor inflammation when you've already had a heart attack. If your CRP level remains high, your chance of having another heart attack is higher than in someone with a normal CRP level.

Risks

There are very few risks with blood tests. You may experience bruising, swelling, or a hematoma (a solid swelling of pooled blood under your skin) after you have your blood drawn, or you may feel dizzy, lightheaded, or faint during the procedure. As with any entrance wound, there is a slight risk of infection from the needle puncture.

Before the Test

Let your doctor know about any medications you're taking since some kinds can increase or decrease your CRP level.

Timing

A blood draw usually takes less than five minutes. You may have to wait a bit for your turn beforehand, but you will be able to leave as soon as the test is complete, as long as you're not feeling faint or sick.

Location

The location of your test will depend on your doctor. You may have it in the lab at your doctor's office, at a local hospital lab, or somewhere else your doctor indicates. You may even have your blood drawn in the same room in which you had your exam soon after you see your doctor.

What to Wear

You don't need to wear anything in particular, but avoid tight sleeves since you will need to push or roll one up for the test. It's helpful to wear a short-sleeved shirt.

Food and Drink

A CRP or hs-CRP test doesn't require any fasting beforehand, so you may be able to have it right away after you see your doctor. An ESR test doesn't require fasting either. However, many cholesterol tests do, so if your doctor is having your levels tested too, you may need to avoid food and drink for a specific period of time before your test. Your doctor will give you specific instructions on what to do in the event that you're having other tests at the same time.

Cost and Health Insurance

A CRP test is inexpensive (around $12 to $16). If you have health insurance, it should cover this test. But if you're unsure or you have any questions, call the number provided on the back of your insurance card.

What to Bring

If you think you may need to wait for your test, you can bring along some activities to pass the time. You may want to have your health insurance card along, especially if you're having your test done at a different facility than your doctor's office.

During the Test

A lab technician, nurse, or phlebotomist, a person who's trained to draw blood, will perform your CRP test.

Pre-Test

You may have to fill out some routine paperwork before your test. The receptionist will let you know once you check in.

Throughout the Test

The test normally takes just a few minutes. Once you're called into the lab, you'll sit in a chair or on an exam table. The technician will ask you which arm you want to use and have you roll up your sleeve, if necessary.

After the technician finds a vein from which to draw, typically on the inside of your arm in the crease of your elbow, you'll have a band tied around your arm above it to help push the blood down into your vein. The technician will clean the area with alcohol to get rid of any germs that might be on your skin.

This is the point at which you may want to look away, especially if you're squeamish or tend to feel lightheaded, dizzy, or faint around blood and/or needles. The technician will then insert a small needle into your vein. This may feel like a sharp pinch or poke, but the sensation is very brief. Your blood will then be drawn into a tube, the band will be taken off, and when enough blood has been collected, the needle will be removed. The technician will use a cotton ball or tissue to put pressure over the entrance site, especially if you're on a blood thinner like Coumadin (warfarin). If it doesn't stop bleeding right away, you may have a bandage placed over the area.

Be sure to tell the technician if you have a history of fainting during medical procedures or if you begin to feel like you're going to pass out while your blood is being drawn. The technician may have you lie down to prevent you from falling.

Post-Test

Once your bleeding has stopped or you have a bandage placed, as long as you aren't feeling dizzy or faint, you will be able to leave. You may need to sit for awhile after the procedure if you experienced lightheadedness or you fainted.

After the Test

When you're finished having your blood drawn, you can resume your normal activities right away.

Managing Side Effects

Though you may experience some swelling, bruising, pain, or a hematoma (pooling of blood in the skin) in the area in which your blood was drawn, these side effects should be minor and usually go away within a few days. If they don't go away or they get worse, be sure to call your doctor.

Interpreting Results

Depending on where your blood is sent, the results of your CRP test may take a day or two to come back.

CRP Test

As a general rule, there is very little CRP detectable in normal blood, though the levels tend to go up slightly with age and be a little higher in females and African Americans.

For the regular CRP test, a normal reading is under 10 mg/L.

If your results are over 10 mg/L, this usually indicates that you have a severe infection or an inflammatory disease.

Chronic inflammatory disease: If you have been diagnosed with a chronic inflammatory disease, your CRP test result can help your doctor determine how you're doing. For example, if your CRP level is high, you may be having a flare-up, or it could mean that your treatment isn't working as well as expected and thus needs to be tweaked. If your CRP level is low but was previously high, this indicates that your treatment is working and inflammation is subsiding.

Additionally, if your doctor suspects that you have a chronic inflammatory disease like rheumatoid arthritis or lupus but you haven't been diagnosed with one, your CRP test results can help rule this out if they're negative or confirm that your doctor may need to do some more tests if they're positive.

Infection: When your doctor suspects an infection and your CRP test result is positive, this warrants further exploration to determine what's causing your infection and where it is (assuming it's not obvious). If your CRP level has gone down after treatment for an infection, this shows that you're responding to the treatment.

Other causes: Your doctor may do some additional testing if your CRP test was positive to look for other causes of inflammation if the cause isn't obvious. Other conditions that can cause elevated CRP levels include:

hs-CRP Test

The hs-CRP test is used to measure your risk of developing heart disease, which the American Heart Association classifies as follows:

  • Low risk: Lower than 1.0 mg/L
  • Average risk: 1.0 and 3.0 mg/L
  • High risk: Above 3.0 mg/L

The hs-CRP accurately detects lower, more specific levels of CRP than the regular test can, which is why it's used to evaluate heart disease risk.

Follow-Up

CRP test: If you have a chronic inflammatory disease, your doctor will likely do CRP tests regularly to monitor your progress, flare-ups, and treatment success. You may have another test done when you have an infection as well to make sure the treatment is working.

If you have another medical condition that's causing a positive CRP test result, like one listed above, your doctor will work with you to diagnose and treat it. You may have additional CRP tests to monitor the amount of inflammation in your body.

hs-CRP test: Because your CRP levels can fluctuate, your doctor may want to repeat the hs-CRP in two weeks to come up with an average level between the two. This can give a clearer picture of your heart disease risk. If your CRP level is high, you may have other tests done to look at your risk further.

You may also have repeat hs-CRP tests done in the future to monitor inflammation levels if you're at an increased risk for heart disease since you can get your CRP levels down through lifestyle changes and/or medications.

Other Considerations

Be sure to let your doctor know if you have any questions or concerns about your CRP test results. If you want to get another CRP test done, talk to your doctor about how to do so.

A Word From Verywell

Medical tests often come with some amount of anxiety. Thankfully, CRP test results shouldn't take too long, so your waiting time will likely be minimal. Keep in mind that this test simply indicates inflammation, but not what's causing the inflammation or where it is. This means that further testing may need to be done, depending on any underlying medical conditions and the reasons for your test.

View Article Sources