Rapid plasma reagin (RPR) is a blood test to screen for syphilis. Antigens used in RPR test are similar to VDRL except that they are charcoal coated to enhance the sensitivity of the test. Syphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum. RPR is used as a screening test for Syphilis & for following the progression of disease and response to therapy.