C-reactive protein test

Also called the CRP test

Key points about the C-reactive protein test

  • The C-reactive protein (CRP) test is a blood test used to check for the presence of inflammation or infection in your body.
  • C-reactive protein (CRP) is a protein made by the liver and released into the blood in response to inflammation.
  • It plays an important role in the immune process. 
Woman with arm resting on table having blood drawn by healthcare worker
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C-reactive protein (CRP) is a protein made by the liver and released into the blood in response to inflammation. It plays an important role in the immune process. 

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,(external link) inflammatory bowel disease (eg, Crohn's disease(external link) or ulcerative colitis(external link)), or if you have an infection such as pneumonia.(external link)

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. It peaks within 24–72 hours, and returns to normal 2–3 days after the inflammation or infection has stopped.

To diagnose and monitor 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 your treatment is working. If your treatment is working well, and the inflammation lessens, the level of CRP in your blood will drop. 

To diagnose and monitor infection

If you have symptoms of an infection in 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 decide whether or not to prescribe antibiotics. 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 by 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.

You don't need to do anything before having this test, unless it is combined with a test being done for another reason. Your doctor will advise you on this. The CRP test can be done at any time of the day. 

Regular CRP test

A regular CRP test is usually done at your local blood collection centre.(external link)(external link)(external link) 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.

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. 

Key facts about CRP (see references below)

  1. CRP shows a rapid response to infection and inflammation, increasing within hours of stimulus and returning rapidly to normal following resolution. 
  2. Typically CRP begins to rise within 4-6 hours of stimulus, peaks within 36–50 hours, and returns to normal 3–7 days following resolution.
  3. There are distinct ranges of normal and abnormal in CRP reference ranges, without variations for age or gender.
  4. CRP is not affected by conditions such as pregnancy, intercurrent drug use, anaemia and plasma protein variations.

CRP as an indication of severity

When a patient is ill but the diagnosis is not known, the level of CRP may provide additional information to assist in reaching a diagnosis. As the CRP levels rises significantly higher, more urgent review of the patient becomes necessary.    

CRP (mg/L) Severity
10–40 Mild inflammation, viral or bacterial infection
40–100  Moderate inflammation, viral or bacterial infection
100–200  Marked inflammation, bacterial infection
 >200  Severe bacterial infection or extensive trauma


CRP is not suitable as a screen in asymptomatic patients. It should only be requested on patients in whom the clinical evaluation has given some indication of a disease process. Useful indications are as follows:

Scenario Description
When you suspect a significant bacterial infection CRP can be helpful in differentiating between bacterial and viral infections. As the CRP increases above 100mg/L, the likelihood of a bacterial infection becomes greater than viral infection.

CRP can also be useful to assess the severity of a respiratory tract infection (RTI) – is this RTI more serious than it seems?
Response to therapy (antibiotics or steroid therapy) CRP is useful to assess whether the infection has responded to an antibiotic, or to assess whether the patient is responding to a trial of steroid therapy.
Monitoring polymyalgia rheumatica (PMR), temporal arteritis/GCA and rheumatoid arthritis (RA) While a measure of CRP alone is not helpful in the diagnosis of these conditions, it is recommended for monitoring of disease progression. In RA, it is known that sustained high levels of CRP are associated with worse outcomes.

The role of CRP measurements is unclear in the following situations, and is unlikely to be helpful:

  • screening asymptomatic patients
  • as a cardiovascular disease risk factor 
  • in the management of cancer patients (other than in cases of intercurrent infection).


  1. Is point-of-care CRP testing useful in guiding antibiotic prescribing in patients with respiratory tract infections?(external link) BPAC, NZ, 2015
  2. CRP vs ESR(external link) BPAC, NZ, 2005  
  3. C-reactive protein – how to use CRP testing in general practice within consultations for acute respiratory tract infections(external link) General Practitioner Antimicrobial Stewardship Programme Study
  4. Huang Y, Chen R, Wu T, et al. Simple blood test for pneumonia can help limit use of antibiotics(external link) NICE, UK, 2014 
  5. Cals J, Butler C, Hopstaken R, et al.Association between point-of-care CRP testing and antibiotic prescribing in respiratory tract infections  a systematic review and meta-analysis of primary care studies(external link) Br J Gen Pract 2013;63:e787–94.
  6. Jones CH, Howick J, Roberts NW, et al. Effect of point of care testing for C reactive protein and training in communication skills on antibiotic use in lower respiratory tract infections  cluster randomised trial(external link) BMJ 2009;338:b1374.
  7. Primary care clinicians’ attitudes towards point-of-care blood testing – a systematic review of qualitative studies.(external link) BMC Fam Pract 2013;14:117.
  8. Wakeman M, Cork T, Watwood D. Point-of-care C-reactive protein testing in community pharmacy to deliver appropriate interventions in respiratory tract infections(external link) Clinical Pharmacist, 2018
  9. Effect of point-of-care C-reactive protein testing on antibiotic prescription in febrile patients attending primary care in Thailand and Myanmar – an open-label, randomised, controlled trial(external link) The Lancet, 2019 Jan 1;7(1): 

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Sarah Hyder, POAC Regional Nurse Advisor, Auckland, Gwenda Lawrence, medical laboratory scientist, Auckland

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