The FTA-ABS test for syphilis. Performance in 1,033 patients.

During the 20th century, screening tests using nontreponemal lipoidal antigens have evolved from the old Wassermann test to such excellent ones as the VDRL and Kahn flocculation tests and the Kolmer complement-fixation test. These tests are easily performed in small laboratories and are almost invariably reactive in patients who have any form of active syphilis other than primary syphilis. Although the cardiolipoidal antigen tests are sensitive, they are not specific for syphilis, and other clinical or laboratory confirmation is therefore needed. The TPI test has been the standard confirmatory serological test since the early 1950s. Its sensitivity has been proved repeatedly. Its specificity also is excellent because the antigen used in the test is the aetiological agent of syphilis. However, the technical difficulty and expense of TPI testing has limited its use to a very few specialized laboratories, and has prompted a search for other confirmatory tests that are also highly specific and sensitive but more easily performed. In the search, fluorescent antibody techniques have been the most rewarding. Coons, Creech, and Jones (1941) first described the technique of conjugating antibody molecules with fluorescein. Despite conjugation with fluorescein, an antibody will retain its capacity to react specifically with its antigen. There are both direct and indirect fluorescent antibody techniques. Direct techniques use antibodies labelled with fluorescein as a histochemical stain for identifying their specific antigens, or, conversely, the antigens are used to identify their antibodies (Thomason, Cherry, and Moody, 1957; Coons and Kaplan, 1950). Indirect techniques (which are more suited to serological testing) apply unidentified, unlabelled antibodies to known antigens or vice versa (Coons, 1951; Harris, Deacon, and Smith, 1957). After the antibodies have had sufficient time to combine with the antigen, a washing procedure removes all but the attached antibodies. The next step is the addition of fluorescein-labelled antiglobulin which will attach to antibody globulin. Fluorescence proves that an antigen-antibody reaction has taken place. The antibody is therefore identified indirectly. The principle that Coons, and others (1941) described has been successfully applied to many facets of microbiology and immunology. The outcome of its application to serological testing for syphilis has been the development at the Venereal Disease Research Laboratory of a test that rivals the TPI for both specificity and sensitivity. Dr Deacon, who is now director of that laboratory, and his co-workers developed the original fluorescent treponemal antibody test, an indirect technique (Deacon and Hunter, 1962; Deacon, Falcone, and Harris, 1957). Its major steps are as follows: