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CRP blood test

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 2 minutes read

Blood tests have many different uses. They can help diagnose a condition, monitor a particular organ in the body such as the kidney or liver, and they can also be used to give measurements of bodily processes such as blood sugar or current levels of inflammation.

What does CRP measure?

CRP, as it is commonly known, stands for C-reactive protein, which is a protein made by the liver in response to tissue injury or inflammation, and also rises in response to an infection. It is a good test to show the level of infection or inflammation, but it doesn't indicate where it is coming from. If you cut your toe, had a heart attack, had a flare of rheumatoid arthritis, or had the flu, your CRP levels would all rise above normal in response.

Why is my doctor ordering it?

If your doctor is ordering a CRP test as part of your blood tests, it is because they want to get some information on levels of infection or inflammation. If you visited your doctor with flu symptoms, for example, a normal CRP level would be reassuring that there was not a serious amount of inflammation or infection. By itself, the test is difficult to interpret, it needs to be matched with your symptoms, and it may prompt the need for more tests to determine the cause.

The level of CRP can also point to how much inflammation is present. Once treatment is started - for an autoimmune condition, for example - the level can be used to guide how effective a treatment is over time. You would expect the anti-inflammatory medication to reduce these levels if it's working as it should. Hopefully, your symptoms will also be improved.

How quickly does CRP rise?

The CRP increases very quickly after an infection or inflammation. It, therefore, indicates levels of infection or inflammation at a short-term snapshot in time. The levels would change day-to-day. The levels of CRP also drop very quickly when the inflammation or infection starts to get better. For this reason, doctors sometimes use it as a test to monitor how an infection progresses and how effective a treatment has been.

This compares to erythrocyte sedimentation rate (ESR), another marker of infection, which rises only after a few days but continues to be elevated during an inflammation phase. It's, therefore, less useful for acute infection and better for long-term inflammatory conditions.

Are there any special requirements for the blood test?

You do not need to be fasting for this blood test, there are no special requirements. It can be taken like any normal blood test.

Remember to press hard for a good few minutes after the needle has been removed and keep your elbow straight, to prevent a nasty bruise!

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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