CRP value after training: recovery or infection?

The assay counts how much C-reactive protein your blood carries, in milligrams per litre (mg/l). It is a protein your liver produces on a signal from interleukin-6, and that production starts quickly: within six to eight hours of the start of an inflammation the amount is already climbing sharply. The result says how much CRP is circulating and therefore how large the response is, and not where it sits or what caused it.

In athletes interleukin-6 has two sources. One is an infection. The other is the muscle damage of a hard session, because repairing muscle tissue gives off exactly that same signal. That is why this value is awkward to read inside a training panel: the number sees no difference between recovering from a heavy block and getting ill. Heavy eccentric work, a race or an unaccustomed load lift CRP within a day, and after a marathon values in the tens of mg/l at 24 hours are ordinary.

So what you measure is the size of a repair process, and not its quality. Bacterial infections usually give the highest values, far higher than a hard session, but the results overlap too much to read the cause off the number. This page shows the routine assay, with 10 mg/l as the upper limit; the lab reports anything under the test's detection limit as less than 5.

For an athlete the value of CRP sits in the trajectory and not in one result. That is down to the fixed half-life of about 19 hours: once the stimulus is gone, the value halves each day, and it makes no difference whether that stimulus was a session or a virus.

There is a usable distinction in that. Training-induced CRP peaks around 24 to 48 hours after the session and then falls away neatly along that pattern. A value still high after three or four easy days does not fit muscle repair, and the same goes for a value rising while you are not training. In both cases you look for another explanation, together with your doctor.

Chasing a low CRP during a build block is a category error. A raised CRP there is the expected price of the load and no sign that something is wrong. Steering by it as a performance metric means, in practice, training less to make a number look better.

For gauging cardiovascular risk the routine assay is not sensitive enough, because that question plays out below 5 mg/l, where this test no longer looks. The hs-CRP assay exists for it, the same protein with a more sensitive method, and it belongs in a rest period.

Timing matters more for this value than for almost any other in an athlete's panel. Draw at least three to four days after your last hard or unaccustomed session, preferably after a rest day. Do otherwise and you measure the recovery from that session rather than your baseline. After a race or a marathon that window is longer: give it at least a week before treating a result as representative. After an infection the ordinary rule applies, at least two weeks after you have fully recovered.

What a raised result means depends on the moment. A day after heavy eccentric work it is muscle damage and repair, and you re-test after rest. A day after a race it is expected and can run to tens of mg/l; wait a week. If the value is still high after three rest days, it no longer fits muscle repair and you discuss it with your doctor. Mid-deload you have a usable baseline instead, which you set beside earlier measurements. And a value rising without any training calls for another explanation and therefore a conversation with your doctor.

If you want to follow the course of an infection rather than your baseline, a second measurement after 24 to 48 hours is more informative than the first number. An autoimmune disease often raises the value as well, usually for longer and less high. If your CRP is raised, the question is always what raised it; laboratories also do not all use the same upper limit, so read the limit printed on your own result.