The C-reactive protein test

Also called the CRP test

The C-reactive protein test is a blood test which measures the level of a protein in the blood called C-reactive protein (CRP). The level of CRP is used as a marker to check the presence of acute inflammation or infection in the body.

Key points

  • C-reactive protein (CRP) is also known as an acute phase protein.
  • The level of CRP increases when you have certain diseases which cause inflammation. It can also rise if you have an infection.
  • The CRP test may be requested:
    • When your doctor suspects that you might be suffering from an inflammatory disorder. It does not show what is causing the inflammation or where the inflammation is located – it merely confirms the presence of inflammation.
    • If you already have an inflammatory disorder, to monitor the disease, and to determine how well a treatment is working. If a treatment is working well, and the inflammation lessens, the level of CRP in the blood will drop.
    • If you have an infection, the CRP test can help your doctor decide if you need antibiotics or not. 
  • To perform a CRP test, a blood sample is drawn from a vein in the arm, and collected in a tube. This tube is sent to the laboratory for analysis. In some cases your doctor may perform a CRP test during the consult, in their surgery, using a finger-prick blood sample. This is called a point-of-care CRP test.

What is a C-reactive protein test?

The C-reactive protein test measures the level of a protein in the blood called C-reactive protein (CRP). The level of CRP increases when you have certain autoimmune diseases which 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 <5 mg/L; this will begin to rise 4 to 8 hours after tissue is damaged, peak within 24 to 72 hours, and return to normal 2 to 3 days after the inflammation or infection has stopped.

When is the CRP test done?

Inflammation

The CRP test is done to establish the presence of inflammation in the body and is requested when your doctor suspects that you might be suffering from an inflammatory disorder. It does not show what is causing the inflammation, or where the inflammation is located. It is requested:

  • To help in the diagnosis of inflammatory conditions such as arthritis, inflammatory bowel disease, or other autoimmune disorders.
  • If you already have an inflammatory disorder, to monitor the disease, and to determine how well a treatment is working. If a treatment is working well, and the inflammation lessens, the level of CRP in the blood will drop. 

Infection

If you have symptoms of an infection of the 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. The need for antibiotics is decided as follows:

  • Antibiotics are only effective against infections caused by bacteria and are not effective against infections caused viruses.
  • Respiratory tract infections caused by bacteria result in a greater rise in CRP as compared with infections caused by viruses. 
  • A CRP level < 10 mg/L suggests that a patient does not have a bacterial infection and therefore a prescription for an antibiotic is likely to do more harm than good.
  • It is 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 do not need to do anything before having this test, and it can be done at any time of the day. 

How is the sample collected for testing

A blood sample is taken by a needle placed in a vein in the arm. An elastic band is wrapped around your upper arm. It may feel tight for a few seconds. 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.

In some cases, your doctor may perform a CRP test during the consult, 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 analyzed 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 the blood suggests acute inflammation or infection. As the inflammation or infection lessens, the CRP level drops. 

Learn more

The following links provide further 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. Last reviewed: 16 May 2016