A rapid plasma reagin (RPR) test is a blood test used to screen you for syphilis. It works by detecting the nonspecific antibodies that your body produces while fighting the infection. Syphilis is a sexually transmitted infection (STI) caused by the spirochete bacterium Treponema pallidum. Rapid plasma reagin (RPR), a syphilis blood test that looks for antibodies to the syphilis bacteria. Antibodies are proteins made by the immune system to fight foreign substances, such as bacteria.