Treponema pallidumSurface Immunofluorescence Assay for Serologic Diagnosis of Syphilis

A surface immunofluorescence assay (SIFA) using live spirochetes was analyzed and compared with Western blot (WB), fluorescent treponemal antibody absorption (FTA-ABS), microhemagglutination (MHA-TP), and Treponema pallidum immobilization (TPI) assays for detecting serum antibodies to T. pallidum in patients with syphilis, in disease controls, and in healthy subjects. SIFA and WB were 99% sensitive (99 of 100 positive specimens) and specific (140 of 140 negative specimens); FTA-ABS showed a sensitivity and a specificity of 90 and 89% (90 of 100 positive and 125 of 140 negative specimens), respectively. MHA-TP showed a sensitivity of 84% (84 of 100 positive specimens) and a specificity of 98.5% (138 of 140 negative specimens). Finally, TPI had a sensitivity of 52% (52 of 100 positive specimens) and a specificity of 100% (140 of 140 negative specimens). The T. pallidum SIFA was therefore highly specific, showing no equivocal reactivities with control sera, and sensitive. The results suggest the possible use of SIFA as a confirmatory test in the serologic diagnosis of syphilis. The diagnosis of syphilis depends on clinical findings, examination of lesions for treponemes, and/or serologic tests. Serologic tests are divided into nontreponemal and treponemal tests. Nontreponemal tests are useful for screening, while treponemal tests are used as confirmatory tests. In practice, two or three assays are performed in series for the serologic diagnosis of syphilis (22, 28). First, sera are screened in a microflocculation assay, such as the Venereal Disease Research Laboratory test (VDRL), using nontreponemal antigens (25). Reactive sera are then tested for antibodies specific for Treponema pallidum antigens by using the fluorescent treponemal antibody absorption assay (FTA-ABS) or the microhemagglutination assay (MHA-TP) (12, 22, 28). Since the mid-1980s, the Western blot assay (WB), set up with boiled sodium dodecyl sulfate extracts of T. pallidum, has been introduced as an alternative to the other confirmatory methods (5, 13, 15). In addition, among the treponemal tests, several enzyme immunoassays have been described and evaluated, showing sensitivities and specificities similar to those of FTA-ABS and MHA-TP (9, 11, 23, 37). Currently, the T. pallidum immobilization test (TPI), the first treponemal antibody test developed (by Nelson and Mayer in 1949 [26]), is limited to mostly research purposes, since it is complex and technically difficult. The test is based on the ability of the patient’s antibody to bind to the surface of T. pallidum (2) and to immobilize living spirochetes in the presence of complement, as observed by dark-field microscopy. TPI is accepted as a specific test for syphilis, though it is less sensitive than the newer treponemal tests (30). Previous observations from our laboratory showed the high specificity of surface immunofluorescence assay (SIFA) performed with live Borrelia burgdorferi spirochetes for the detection of serum-specific antibodies against surface antigens of the Lyme disease agent (7). In this study, we investigated the sensitivity and specificity of SIFA both for the detection of specific antibodies against T. pallidum and for the serologic diagnosis of syphilis.