CRP blood test: normal range explained

CRP measures the level of C-reactive protein in your blood. Your liver makes CRP in response to inflammation, so the level rises within hours of an infection, an injury or a flare of an inflammatory disease, and falls again as it settles.

The test shows how much inflammation is present, not where it is or what is causing it. A standard CRP is different from a high-sensitivity CRP (hs-CRP), which measures very small amounts and is used to estimate heart disease risk; the two are sometimes confused on reports.

Doctors order CRP to help check for infection, with symptoms such as fever, chills or a fast heart rate, and for inflammatory conditions such as inflammatory bowel disease, rheumatoid arthritis, lupus and vasculitis. It is also used after surgery to watch for infection and to guide treatment in sepsis.

CRP is repeated to see whether treatment is working: MedlinePlus says a falling level suggests you are healing. It is often done with ESR, which MedlinePlus says also looks for inflammation, and your doctor reads it with other tests, such as the WBC count.

CRP responds to more than illness, and a few things can nudge it without any disease.

• Smoking, obesity, insomnia, depression and menopausal hormone therapy are linked to higher CRP.
• Hard exercise, such as a long run or heavy weight training, before the test can raise it, as can a recent illness or injury.
• Pregnancy, in its second half, and birth control pills can give a positive CRP.
• Medicines such as NSAIDs (ibuprofen, aspirin) and magnesium may affect the result, and statins may lower it.
• Levels rise slightly with age and tend to be a little higher in women.
• Tell your doctor about all your medicines and supplements; do not stop a prescription medicine without talking to them first.