Fluorescent treponemal antibody inhibition test.

The fluorescent treponemal antibody (FTA-200) test has been shown to detect at least two types of antibody in syphilitic serum; one of these, which can be removed by absorption with Reiter treponemes, is thought to be a group antibody which reacts with an antigen shared by Treponema pallidum and other treponemes (Deacon and Hunter, 1962). A second antibody, which remains after such an absorption, is thought to be specific for T. pallidum. The group antibody is present in many nonsyphilitic sera in low titre, possibly being produced in response to the presence of commensal treponemes; it rarely reaches a titre of 1 in 200, the dilution at which sera are tested in the FTA-200 test. Wilkinson and Rayner (1966) have shown that, except in early syphilis, the group antibody preponderates in syphilitic sera. Non-specific reactions may occur in the FTA-200 test apart from the presence of an unusually high titre of group antibody in non-syphilitic sera; Fife (1964) reported such reactions with some sera containing abnormal macroglobulins and Wilkinson and Rayner (1966) demonstrated that sera containing rheumatoid factor could give positive FTA tests if the treponeme suspension used was partially sensitized with antibody from the rabbit from which it was prepared. Absorption procedures with intact Reiter treponemes, a sonicate of these organisms, or heated culture filtrates (Hunter, Deacon, and Meyer, 1964; Hunter, 1964; Betz, 1966; Deacon, Lucas, and Price, 1966) have been claimed to improve the sensitivity and specificity of the FTA test by removing the group antibody from sera. It has not been shown that these procedures will abolish reactivity of sera containing abnormal globulins in the FTA test. Inhibition of the fluorescence of antigen with fluorescein-labelled antibody by first saturating the antigen receptors with unlabelled antibody of known specificity has been widely used