CRP (C reactive protein)

CRP (C-reactive protein) is a protein produced by the liver during inflammation, infection or tissue damage. Blood testing shows whether inflammation is present and how pronounced it is; repeat testing helps assess treatment response. It does not show where inflammation is or what caused it.

Who is this test for

• people with fever or suspected infection, most often alongside complete blood count, when a doctor assesses whether antibiotics are needed;
• people treated for infection, such as pneumonia or urinary tract infection, to assess treatment response;
• people with chronic inflammatory diseases, such as rheumatoid arthritis or Crohn’s disease;
• people after surgery when a doctor monitors recovery and possible complications;
• people with persistent unexplained symptoms, such as fatigue, fever, joint pain or weight loss.

If someone with fever is confused, breathes very rapidly or has a rash that does not fade under pressure, do not wait for results; call emergency medical services immediately.

What CRP shows

CRP is an acute-phase protein: healthy people have very little in blood, but when inflammation begins, the liver rapidly produces much more. It rises within approximately 6 hours, peaks after around 48 hours and may increase more than a thousandfold during severe inflammation. When inflammation subsides, CRP falls rapidly, making it useful for monitoring treatment.

The test is sensitive but non-specific. CRP rises in bacterial and viral infections, after injury, burns and surgery, in rheumatic and inflammatory bowel diseases, and in certain malignancies. It may also be mildly high in people with obesity and smokers. Results are therefore always interpreted alongside symptoms, examination and other tests.

Compared with ESR CRP rises and returns to normal faster, reflecting acute changes better. Read more about how inflammatory markers are used together in the article Inflammatory parameters and what they tell us.

Panels including this test

Test CRP (C-reactive protein) is also included in this panel.

Children’s panelCBC, CRP, Urine: dipstick and sediment22.00€Details ›

All panels including CRP (C-reactive protein) (9) ›

Preparation and sample

A venous blood sample is taken; blood collection in children is adapted to their age. Fasting is not required for CRP. If tests requiring fasting, such as glucose or blood lipids, are also taken that day, come fasting in the morning. Mention antibiotics, corticosteroids or medicines affecting immunity at blood collection, because they may lower CRP.

How results are interpreted

CRP is expressed in milligrams per litre (mg/L). In healthy adults it is usually below 5 mg/L; the exact cut-off depends on method and is printed on the report. The number alone cannot identify a disease, but the CRP level guides a doctor:

• Mildly high CRP, up to approximately 10 mg/L, commonly results from mild viral infection, chronic disease, obesity or smoking.
• Moderately high CRP, several tens of mg/L, occurs in both viral and bacterial infections and inflammatory diseases.
• Values above 100 mg/L more often accompany bacterial infection, but also severe inflammation, major injuries and surgery.

CRP helps distinguish viral from bacterial infection but cannot decide this alone. Some viruses, such as adenoviruses, may raise it substantially, while CRP may still be low at the very start of a bacterial infection. A doctor therefore also considers white blood cell numbers and types in the blood count and differential blood count, repeating testing after a day or two if needed.

Normal CRP does not exclude disease: it may be low in the first hours of infection or infections confined to one site, and in systemic lupus erythematosus it is often normal even with active disease. When monitoring treatment, the trend matters more than one result: falling CRP after several days shows treatment is working. A doctor interprets results alongside symptoms and other findings.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contactpage). Prices for all tests and panels are listed in the service price list. For children, the Children’s Panel, including a complete blood count and urinalysis alongside CRP, is practical.

Frequently asked questions

What is a normal CRP value?

In healthy adults, CRP is usually below 5 mg/L. The cut-off depends on the method and is stated alongside the result, so comparing reports from different laboratories by numbers alone is not advisable.

Can CRP be high in a viral infection?

Yes. In most viral infections, CRP is normal or moderately high, but some can raise it enough to resemble bacterial infection. Antibiotic decisions are therefore not based on CRP alone.

Why is CRP tested alongside a complete and differential blood count?

CRP shows how pronounced inflammation is, while complete and differential blood counts show how white blood cells respond. Raised neutrophils with high CRP more often accompany bacterial infection, and raised lymphocytes viral infection, giving a fuller picture together.

What does mildly high CRP without symptoms mean?

Most often a recent mild infection, injury or chronic low-grade inflammation, for example with obesity or smoking. A doctor usually recommends repeat testing after several weeks and further investigations if CRP remains high or symptoms develop.