Treponemal haemagglutination test.

The introduction of the treponemal immobilization (TPI) test by Nelson and Mayer (1949) provided the first verification test for syphilis which proved fully acceptable to the clinician. After 20 years it is still accepted by a large body of opinion as the verification test of choice for the diagnosis of latent and late syphilis. The test's characteristic of becoming positive towards the end of the primary or in the early secondary stage of the disease, after the classical tests and the Reiter protein complement-fixation (RPCF) test, requires that a further specimen be examined after a few weeks before the diagnosis of a biological false positive (BFP) reaction can be accepted. The technical difficulties of the TPI test are numerous, many stemming from the need to use freshly-harvested living treponemes for each batch of tests. The introduction of the fluorescent treponemal antibody (FTA) tests removed this problem, but until the development of the FTA (Absorbed) test, these procedures lacked either sensitivity or specificity. While the FTA Absorbed (FTA-ABS) test is replacing the TPI test in many centres, the limiting factor in their application is that both are read microscopically. The publication by Rathlev (1965, 1967) of a haemagglutination test utilizing pathogenic Treponema pallidum for the serology of syphilis, provided the first practical serological test using these organisms which could be read with the naked eye. It was decided to investigate this reaction, which was based on the agglutination of tanned formolized sheep erythocytes sensitized with a sonicate of washed Nichols strain Treponema pallidum. Experiments were carried out to define the conditions for the test and these confirmed Rathlev's findings, as did the work of Tomizawa and Kasamatsu (1966). The results of the preliminary examination of the test were satisfactory, and it was decided to modify the test so that it could be applied to all specimens received by the laboratory. The need to absorb all specimens before testing was eliminated by replac-