Absorbed fluorescent treponemal antibody (FTA-ABS) test. Comparison with the FTA-200 and TPI tests on 1,056 problem sera.

Deacon and Hunter (1962) have shown that the fluorescent treponemal antibody (FTA) test detects two separate antibodies in syphilitic sera. One, a group antibody, reacts with other treponemes, such as the Reiter organism, Treponema microdentium, and Treponema zuelzerae, as well as with Treponema pallidum, presumably because of an antigen shared in common by these organisms. Removal of group antibody from syphilitic sera by absorption with Reiter treponemes allows the detection of a second antibody which is thought to be specific for T. pallidum. Group antibody is present to low titres in the majority of normal, non-syphilitic sera, possibly being produced in response to the presence of commensal treponemes in the body. Wilkinson and Rayner (1966) have shown that it also occurs in the course of syphilitic infection and that, except in secondary syphilis, it appears to preponderate over the antibody specific for T. pallidum. This has also been found to occur in infections with yaws (Wilkinson, unpublished data). In the FTA-200 test described by Deacon, Freeman, and Harris (1960), sera are tested at a dilution of 1 in 200 which is above the threshold of group antibody found in the great majority of non-syphilitic sera. Despite the sensitivity of the indirect fluorescence technique, this dilution reduces the potential sensitivity of the method and various means have been advocated for the removal of group antibody from sera so that tests can be made for the presence of the antibody specific for T. pallidum alone. This has been done by absorption of sera with intact Reiter treponemes, by a sonicate of these organisms or by a heated filtrate of a culture of Reiter treponemes (Hunter, Deacon, and