TP is a spirochete bacterium with an outer envelope and cytoplasmic membrane. About What Is Syphilis Test?, the Syphilis test is used to screen for and diagnose syphilis spirochetes and is primarily a qualitative test for antibodies to syphilis spirochetes in whole blood, serum or plasma. The syphilis spirochete (TP) is the causative agent of the sexually transmitted disease syphilis.