Which of the following is the most sensitive test for Treponema?

Treponemal tests detect antibody specific to T pallidum, such as an indirect immunofluorescent procedure called the fluorescent treponemal antibody (FTA-ABS), which uses spirochetes fixed to slides. ABS refers to an absorption step that removes nonspecific anti spirochetal antibodies often found in normal serum. FTA-ABS is the most sensitive test in all stages of syphilis, and is the best confirmatory test for a serum reactive to a screen such as RPR or VDRL. FTA-ABS antibodies rise more quickly in primary syphilis and remain positive in teiary syphilis. In late syphilis, approximately 1/3 of these patients may have a nonreactive VDRL or RPR. Therefore, if there is clinical suspicion of late syphilis and the VDRL or RPR is nonreactive, the FTA-ABS should be performed. This test is both sensitive and specific for syphilis infection. Less than 1% false positives are due to SLE, RA or old-age. All 1+ or minimally reactive results are considered equivocal; in the absence of historical or clinical evidence of treponemal infection, a second specimen should be submitted for serological testing. Ref: Ray C.G., Ryan K.J. (2010). Chapter 37. Spirochetes. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.