CRP value: how high may your inflammation marker be?

The test measures how much C-reactive protein is in your blood, in milligrams per litre (mg/l). It is a protein your liver makes on a signal from inflammation: as soon as something is going on somewhere, the amount of CRP rises sharply within six to eight hours, and afterwards it falls just as fast once the cause is gone. How much CRP is in your blood says something about the scale of the response and nothing about the place.

It helps to know what the test does and does not do. It tells you that something is going on. It does not tell you where it sits or what it is. An infected tooth, a bladder infection and a large bruise can all three produce the same number. Bacterial infections usually give the highest values and viral ones often stay lower, but the results overlap too much to draw anything from that on your own.

Your result is in mg/l, and up to 10 mg/l counts as normal on the routine assay. The full name is C-reactive protein. A CRP test measures that one protein and nothing else.

The most common question about this value is always the same: my CRP is 12 after a cold, should I worry? Usually not, and that comes down to how fast this protein moves. Your CRP starts rising six to eight hours after the trigger and then roughly doubles every eight hours. It peaks after two days, and after that it halves each day as long as the cause is gone.

So a value of 12 a few days after a cold is not strange. It is exactly what you would expect from a body still clearing up, and it says nothing about anything chronic. The only way to be sure is to re-test once you have genuinely recovered; two weeks after your symptoms have gone is a good rule of thumb. If the value is normal then, you were right. If it is still high, that is a question for your GP, and now it is a useful one.

Your GP uses the same number the other way round, as an aid to a decision. In someone with a cough, the Dutch GP standard takes a CRP under 20 mg/l as a sign that pneumonia is unlikely and above 100 mg/l as a sign that it is likely; in between, the symptoms weigh in. Those are limits for that one decision in the consulting room, and not limits to hold your own result against. For gauging cardiovascular risk the routine assay is also not sensitive enough, because that question plays out below 5 mg/l, where this test no longer looks; the hs-CRP test exists for that.

When you get tested decides what your result is worth. Test too early and you measure the aftermath of something already over. The rule of thumb is simple: test when you feel well and rested, at least two weeks after a cold or flu has completely gone, and a few days after a hard training session.

These are the things that lift your value temporarily without anything being wrong, with the waiting time attached. After a cold or flu you wait two weeks after recovery. After a hard training session three to four days, and after a marathon or race about a week. After a vaccination a few days. After surgery or a large bruise you discuss the timing with your doctor, and with an infected tooth or gums you wait until it is treated.

If you want to know whether an infection is clearing, testing twice with a day in between is more useful than one isolated result, because then you see the direction. The test is there to confirm inflammation or rule it out: acute inflammation such as pneumonia usually gives a clearly high number, and with an autoimmune disease the value often stays mildly raised for longer. There are two CRP tests, and the routine one is what you get here. If your CRP is raised, it tells you that something is going on; the upper limit your lab uses is printed on your result, because not every lab uses the same one.