The Rapid Plasma Reagin (RPR) test is a blood test that screens for syphilis, a sexually transmitted infection (STI) caused by the bacterium Treponema pallidum. The RPR test detects antibodies (reagin) reacting with the cardiolipin-lecithin-cholesterol antigen. The test can be either reactive or nonreactive, depending on whether antibodies were detected. A nonreactive RPR result means there were no antibodies to syphilis found in the blood sample. If you have a history of syphilis and your RPR test is negative or nonreactive, its likely that you no longer have syphilis. A positive RPR test should be followed by another type of test to diagnose syphilis. If you have been treated for syphilis in the past, an RPR test that shows a titer increase of fourfold means you likely have a new syphilis infection if you were not fully treated in the past.