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Reactivity in the FTA-ABS test of rabbits hyperimmunized with nonpathogenic treponemes.

In the absorption step of the fluorescent treponemal antibody-absorption (FTA-ABS) test (USPHS, 1969) sorbent is used to circumvrent so-called natural antiTreponema pallidum (Tp) reactivity present in a significant proportion of presumed nonsyphilitic individuals (Hunter, Norins, Falcone, and Stout, 1968). It has been postulated that the natural reactivity may be stimulated by nonpathogenic, saprophytic treponemes (Deacon and Hunter, 1962). Sorbent is a concentrated, clarified supernatant fluid from an autoclaved Reiter broth culture prepared according to methods described by Stout Kellogg, Falcone, McGrew, and Lewis (1967). The best concentration at which the sorbent is to be used in testing human sera is determined for each lot using two main criteria. The sorbent, when mixed four parts with one part serum, should remove all natural anti-Tp reactivity from a number of standard human nonsyphilitic sera known to have a high degree of such natural reactivity. At the same time, the sorbent should not remove the anti-Tp reactivity in a number of human syphilitic sera. In this report, we present data from an animal study showing that, in an FTA-ABS type of procedure, sera from rabbits hyperimmunized with various nonpathogenic treponemes continue to react with Tp after treatment with sorbent at the concentration routinely used in the FTA-ABS test. The reactivity was eliminated in most of the rabbit sera by absorption with Reiter protein antigen and in all sera by absorption with intact or sonically disrupted Reiter treponemes, both of which have been used as absorbents in earlier versions of the FTA-ABS procedure (Deacon and Hunter, 1962; Hunter, Deacon, and Meyer, 1964).