C-reactive protein is made by the liver in response to inflammation. It rises with infection, injury, autoimmune disease, obesity and cardiovascular risk, and it falls again as the trigger settles. That sensitivity is also its limitation: a raised CRP tells you something is going on without telling you what. MedlinePlus states it directly, that CRP results "tell you how much inflammation you have in your body, but not what's causing it or where it is".

Standard CRP and high-sensitivity CRP

These are the same protein measured with different precision. Standard CRP is used to detect and follow significant inflammation, such as infection or a flare of an inflammatory disease, and is usually reported in whole numbers of mg/L. High-sensitivity CRP measures the low end accurately enough to be used for cardiovascular risk stratification. The 2018 AHA/ACC cholesterol guideline lists an elevated high-sensitivity CRP of 2.0 mg/L or above among the risk-enhancing factors that can move a borderline statin decision, describing it as a factor to consider if measured rather than a test for everyone.

Why context decides the meaning

A CRP of 40 mg/L in someone with a fever and a cough is an infection marker. The same value in a well person is a reason to look for an occult inflammatory process, not a reason to start a supplement. MedlinePlus lists infections, inflammatory bowel disease, autoimmune conditions such as lupus and rheumatoid arthritis, lung disease and smoking among the causes of a raised CRP, and notes that obesity and hormone therapy are associated with higher levels. Testing during or shortly after any illness produces a number that describes the illness rather than the baseline.

What to do with a mildly raised result

For a high-sensitivity CRP between 2 and 10 mg/L in a well person, repeat it in two to four weeks when you are free of infection. If it stays raised, the response is to work on the drivers with evidence behind them: treat the cardiovascular risk factors, treat sleep apnoea, address visceral fat, stop smoking. A persistent CRP above 10 mg/L is not a wellness finding and warrants a search for a specific cause rather than an anti-inflammatory diet.

Result	Usual meaning	Sensible response
hs-CRP below 1.0 mg/L	Low inflammatory burden	No action
hs-CRP 2.0 mg/L or above	Risk-enhancing factor in the 2018 AHA/ACC guideline	Use in the statin discussion
CRP above 10 mg/L in a well person	Not explained by cardiovascular risk alone	Repeat, then investigate a cause
Any CRP during acute illness	Reflects the illness	Retest after recovery

If a CRP has come back high, ask two questions: were you unwell in the two weeks before the draw, and what would change if the value were normal. Repeat the test when you are well before anyone builds a treatment plan on it.

CRP rises within about 6 hours of an inflammatory stimulus, peaks around 48 hours and has a plasma half-life of roughly 19 hours, so it tracks acute change more closely than the erythrocyte sedimentation rate, which lags by days. That difference is why CRP is preferred for following the response to treatment of infection.

Interpretation caveats worth noting: obesity raises hs-CRP through adipose tissue interleukin-6 production, oral oestrogen raises it while transdermal oestrogen has less effect, statins lower it, and intense exercise raises it transiently. Because within-person variability is considerable, cardiovascular risk stratification conventionally uses the average of two measurements taken at least two weeks apart in a person free of acute illness.

Sources

Standard CRP and high-sensitivity CRP

Why context decides the meaning

What to do with a mildly raised result

Sources

Related