The C-reactive protein test

Also called the CRP test

The C-reactive protein (CRP) test is a blood test used to check for acute inflammation or infection in your body.

Key points

  1. The CRP test measures the level of a protein in your blood called C-reactive protein.
  2. The level of CRP increases when you have certain conditions that cause inflammation. It can also rise if you have an infection.
  3. The CRP test is used to diagnose and monitor inflammatory diseases.
  4. It can also be used to check whether antibiotics are needed for an infection.
  5. A CRP test can either be done at a blood collection centre or your doctor may perform a ‘point-of-care’ CRP test using a finger-prick blood sample.

What is a C-reactive protein test?

The CRP test measures the level of a protein in your blood called C-reactive protein (CRP). The level of CRP increases when you have certain autoimmune diseases that cause inflammation, such as arthritis, inflammatory bowel disease (such as Crohn's disease or ulcerative colitis), or if you have an infection such as pneumonia.

The serum CRP level in a “healthy” person is usually less than 5 mg per L. This will begin to rise 4–8 hours after tissue is damaged, peaks within 24–72 hours, and returns to normal 2–3 days after the inflammation or infection has stopped.

When is the CRP test done?

Inflammation

The CRP test can be used to check for inflammation in your body. It is requested when your doctor suspects that you might have an inflammatory disorder. It does not show what is causing the inflammation or where the inflammation is located. It is requested for:

  • diagnosis – to check whether you have an inflammatory condition such as arthritis, inflammatory bowel disease or other autoimmune disorders
  • monitoring – if you have already been diagnosed with an inflammatory disorder, to monitor the disease and see how well a treatment is working. If a treatment is working well, and the inflammation lessens, the level of CRP in your blood will drop. 

Infection

If you have symptoms of an infection of your chest or airways (upper respiratory tract infection) such as dry cough, sore throat, runny nose and sneezing, your doctor may perform a CRP test to assess whether to prescribe antibiotics or not. Infection caused by bacteria results in a greater rise in CRP compared with infections caused by viruses. Antibiotics are only effective against infections caused by bacteria and are not effective against infections caused viruses.

A CRP level less than 10 mg per L suggests that you don't have a bacterial infection and therefore a prescription for an antibiotic is likely to do more harm than good. This is because it's likely that the potential side effects of the antibiotic medication outweigh any clinical benefits.

How to prepare for the test

For the most part, you don't need to do anything before having this test. It can be done at any time of the day. 

How is the sample collected for testing?

CRP test

A regular CRP test is usually done at your local blood collection centre. An elastic band is wrapped firmly around your upper arm. This helps the veins below expand, making it easier to draw blood from. The injection site is cleaned with an alcohol swab before a needle is inserted into your vein.

You may feel nothing at all from the needle, or you may feel a small brief sting or pinch. The blood sample is collected in a tube, which is sent to the laboratory for analysis.

Point-of-care CRP test

In some cases, your doctor may perform a CRP test in their surgery. This is called a point-of-care CRP test. This is a finger-prick blood test where your doctor will gently prick and squeeze your finger for a small blood sample.

The blood sample is collected in a tiny tube, which is analysed by a portable machine in the surgery. The results of the point-of-care CRP test are available within a few minutes.      

Results

On its own, a CRP test rarely provides a diagnosis, but it can confirm the presence of inflammation or infection. An increasing or high amount of CRP in your blood suggests acute inflammation or infection. As the inflammation or infection lessens, the CRP level drops. 

Learn more

The following links have more information on the CRP test. Be aware that websites from other countries may contain information that differs from New Zealand recommendations.

Blood test safety information Labtests NZ
Blood tests to detect inflammation Patient Info, UK
Lab tests Online  Australasian Association of Clinical Biochemists

References

Credits: Health Navigator Editorial Team. Reviewed By: Sarah Hyder, POAC Regional Nurse Advisor, Auckland Last reviewed: 19 Feb 2019