Biomarker 101: C-reactive protein (CRP)

CRP tells you inflammation is present somewhere in the body. It does not tell you where, or why — and that limitation is the key to reading it.

What CRP is

C-reactive protein is produced by the liver and rises in response to inflammation, whether from infection, injury or an ongoing condition. It responds quickly and falls away just as quickly once the cause resolves.

That responsiveness makes it useful. It also makes it easy to misinterpret, because a recent cold will raise it as readily as anything else.

Standard CRP and hs-CRP

Two versions exist, measuring the same protein at different sensitivities.

Standard CRP is used for acute situations — investigating infection, monitoring an inflammatory condition, tracking recovery.

High-sensitivity CRP detects much lower concentrations. It is used to assess the persistent low-grade inflammation associated with cardiovascular risk, which sits well below what a standard test would register.

Acute versus chronic inflammation

Acute inflammation arrives fast and has an obvious cause: an infection, an injury, surgery. CRP rises sharply and settles as things resolve.

Low-grade chronic inflammation is different. It develops slowly, produces no distinct symptoms, and is associated with excess body fat, smoking, inactivity, poorly controlled diabetes and several ongoing conditions. This is what hs-CRP is designed to detect.

What the numbers mean

CRP is typically below 5 mg/L in healthy people, though laboratories differ. For cardiovascular risk assessment, hs-CRP is generally read in bands, with higher values corresponding to greater population-level risk.

Timing matters more here than with most markers. Any recent illness or injury will lift CRP, so a result taken during or shortly after an infection reflects that infection rather than your baseline. Testing once acute illness has resolved gives a far more meaningful number.

What CRP won't tell you

• It doesn't locate anything. CRP indicates inflammation exists, not where it is.
• It doesn't distinguish causes. A chest infection and an inflammatory condition can produce similar readings.
• A single result is easily confounded. Recent illness, injury or surgery all raise it.
• It isn't a risk score. hs-CRP contributes to cardiovascular risk assessment alongside lipids, blood pressure and other factors — it does not stand alone.

The bottom line

CRP is a sensitive, non-specific signal. Its value depends almost entirely on context — what else is happening, when the sample was taken, and what else is on the panel.