The C-reactive protein (CRP) test is used to find swelling and infection in your body. It does this by measuring the amount of CRP in your blood.CRP is a protein made by the liver and sent into the bloodstream. Blood levels may be higher when you have swelling (inflammation) or an infection.he C-reactive protein (CRP) test is used to find swelling and infection in your body. It does this by measuring the amount of CRP in your blood.