When false-positive tests for syphilis occur, doctors request another blood test. The blood-screening tests most often used are either a RPR (rapid plasma reagin) test or a VDRL (Venereal Disease Research Laboratory) test. It is sometimes difficult to interpret blood tests for syphilis...therefore, repeated tests are often necessary to confirm the diagnosis. False-positive VDRL tests for syphilis are common, which is why the confirmatory FTA test — the one that actually shows the presence of the bacterium — is so important.