CRP

Most UK laboratories print an upper limit of 5 mg/L, some 10 mg/L, and report low values as '<1' or '<5'. It is not affected by age or sex to any useful degree.

C-reactive protein is a protein made by the liver that rises sharply within hours when there is inflammation or infection anywhere in the body, and falls quickly again once that settles. It is a general signal of inflammation rather than a pointer to any one cause. The high-sensitivity CRP is the same protein measured on a finer scale at the low end.

Doctors request it very commonly to look for infection or inflammation, to follow the response to treatment of an infection or an inflammatory condition, and as part of the work-up for tiredness, joint pain or unexplained symptoms.

• Bacterial infections, often to very high levels
• Viral infections, usually to a lesser degree
• Inflammatory conditions such as rheumatoid arthritis, inflammatory bowel disease and vasculitis
• Injury, surgery, burns and a heart attack
• Some cancers
• Being overweight, which produces a persistent low-level rise in the general population

• Settling of the infection or inflammation that raised it
• Anti-inflammatory and immune-suppressing medicines, including steroids
• Some biologic medicines, which lower it even when inflammation continues
• Severe liver failure, because the liver makes it

• A cold, a dental infection or a recent vaccination raises it for a few days
• Hard exercise and minor injuries raise it briefly
• It is a little higher in people who are overweight or who smoke, without any illness

No fasting needed. Because it rises and falls quickly, a single value reflects the previous day or two; repeat testing shows whether something is settling.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.