High CRP: what a raised C-reactive protein result means

C-reactive protein is what doctors call an acute phase reactant. Lab Tests Online UK explains that it is made by the liver and released into the blood within a few hours of tissue injury, the start of an infection or other inflammation, and that it is often the first evidence of inflammation or infection, with rising levels frequently coming before pain, fever or other clinical signs.

The concentration can jump a thousand-fold in response to inflammation, which is why it is useful for monitoring. It is a general, non-specific marker. That is its strength, because it picks up a wide range of problems, and its weakness, because the result alone cannot tell you which problem it is.

Doctors request CRP when they suspect an inflammatory disorder, such as certain types of arthritis, autoimmune disorders or inflammatory bowel disease, or to check for infection, especially after surgery. It is also used to assess the severity of inflammation and to see whether treatment is working.

• Inflammatory bowel disease.

• Some forms of arthritis, such as rheumatoid arthritis.

• Autoimmune diseases such as lupus.

• Suspected serious bacterial infection such as sepsis.

• Pelvic inflammatory disease.

• Monitoring for infection after surgery or invasive procedures.

A high or rising CRP suggests that there is an acute infection or inflammation. It does not say where it is or what is causing it. In people who already have a chronic inflammatory condition, a high CRP suggests a flare-up, or that treatment has not been effective. When the result falls below 10 mg/L, Lab Tests Online UK says there is no longer clinically active inflammation.

Your doctor interprets the number alongside your symptoms, examination and other tests. The reference range and units can vary between laboratories, so the range printed on your own report is the one to use.

It is also worth knowing that some things that are not illness can raise CRP, including the later stages of pregnancy, birth control pills and hormone replacement therapy, and that higher levels have been observed in people with obesity.

CRP is rarely interpreted on its own. The NHS lists ESR (erythrocyte sedimentation rate), CRP and a full blood count among the blood tests used when rheumatoid arthritis is suspected, and says no blood test can definitively prove or rule out the condition. The full blood count can also show anaemia, which is common in rheumatoid arthritis but does not prove it.

For polymyalgia rheumatica, the NHS explains that ESR and CRP are used together: if both are normal, polymyalgia rheumatica is unlikely to be diagnosed, and sometimes ESR is normal while CRP is raised, which is more likely to indicate the condition. As inflammation is a feature of many conditions, the NHS says high levels do not automatically mean you have polymyalgia rheumatica.

Lab Tests Online UK says both CRP and ESR rise in inflammation but CRP rises and falls faster, so CRP may return to normal after successful treatment while ESR stays abnormal for longer.

Lab Tests Online UK explains that regular CRP and high-sensitivity CRP (hs-CRP) measure the same molecule. The hs-CRP test detects low-grade inflammation and is used to help assess the risk of cardiovascular disease such as heart attack and stroke, and is not widely used. A regular CRP is the one requested for infection or chronic inflammatory disease. If a result appears labelled hs-CRP, it is being used for a different question.

There is no single next step for a raised CRP, because it depends on why it was measured. If you were unwell with an infection, the test may simply be repeated after treatment to show that it is falling. If there are joint symptoms, a GP may arrange the other tests described above and consider referral to a specialist (rheumatologist), as the NHS describes for suspected rheumatoid arthritis.

If a raised CRP was found in someone without obvious symptoms, your doctor will look for a reason using your history, examination and other tests. Ask your GP what the result is being used to answer, and whether and when it will be repeated.

If you feel acutely unwell, with a high temperature, confusion, difficulty breathing or a rapidly worsening illness, do not wait for a result: use NHS 111 or call 999.