CRP (C-reactive protein): what it is, what it tells you and why we test it

CRP, or C-reactive protein, is a protein your liver makes when there is inflammation in your body. With an infection it rises a lot, within hours. The high-sensitivity version, the one Rosa measures, also reads very low levels: background inflammation, the kind you do not feel.

Why we test it

Inflammation is how your body fights an infection or repairs damage. Once the problem passes, CRP comes back down.

Sometimes, though, inflammation stays switched on at a low level for months or years, without symptoms. This background inflammation is linked to a higher risk for your heart and arteries.

High-sensitivity CRP picks it up. It also helps read other tests: with inflammation, for example, ferritin rises even without too much iron.

What it tells you, and what it does not

If it is high, there is inflammation in your body. But CRP does not say where or why: a cold, a small injury or background inflammation can all raise it.

That is why one high result is not enough: what counts is the average of two draws a few weeks apart, when you are well.

If it is low, that is a good sign: there is little inflammation in your body.

That is why it is read together with other tests:

Normal CRP levels

The lab decides what is normal. For high-sensitivity CRP, many lab reports give below 3 mg/L; some use a slightly higher limit. Your own report may print different limits.

On your report it may also be called ultrasensitive CRP or hs-CRP. Many reports, though, show standard CRP, designed to find infections, often in mg/dL. In mg/dL the number is ten times smaller: 0.3 mg/dL is 3 mg/L.

Both versions measure the same protein. The high-sensitivity one also reads very low levels precisely, the ones that matter for prevention.

In the Rosa report you also see your optimal range and what to do to reach it or stay in it.

What your result means

Find the row for your result, in mg/L. If your report is in mg/dL, multiply by ten first. Which values need a doctor? Above 10, and your doctor usually has it repeated; above 100, the same day.

If your CRP is high or low

What can skew the result

A few things push CRP up for a while. It helps to know them before your blood test.

When to worry

Talk to your doctor if:

With Rosa, a CRP above 10 mg/L is always flagged to a doctor, who usually has it repeated; above 100, urgently.

Frequently asked questions

What are normal CRP levels, in mg/L and in mg/dL? For high-sensitivity CRP, many lab reports give below 3 mg/L, which is below 0.3 mg/dL. For standard CRP the printed limits are often higher. The limit on your own report is what counts.

What is the difference between standard and high-sensitivity CRP? They measure the same protein. Standard CRP is used to find infections and strong inflammation. The high-sensitivity test also reads very low levels, useful for background inflammation and heart risk.

What is the difference between ESR and CRP? Both are tests for inflammation. CRP rises within hours and falls within days once the inflammation passes. ESR changes more slowly and also depends on other things, such as age, anaemia or pregnancy.

Do I need to fast for a CRP test? Not for CRP. It is usually done with other tests that do need fasting, though, so it is best to arrive fasted. You can drink water.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year. If it is high, it is usually repeated a few weeks later, once you are well.

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