Performance and use of the Reiter protein complement-fixation (RPCF) test.

The immunological basis for the use of the RPCF test in the sero-diagnosis of syphilis is the fact that Treponema pallidum and Treponema reiteri share a genus-specific protein antigen. Although the Reiter protein antigen appears to be a lipo-polysaccharideprotein complex (de Bruijn, 1959), it has been assumed that only the protein component is responsible for the reactivity with syphilitic serum. In previous papers we have recommended the application of the RPCF test as a simple, sensitive, and specific test. As is the case with all serological procedures, the sensitivity and specificity of the RPCF test are dependent upon a number of factors, not the least important being the technique by which it is performed. In our experience with many thousands of sera, optimal sensitivity can be obtained utilizing the complement-fixation technique of either de Bruijn (1958) or Kolmer (1942), in which 1 '5 exact units of complement are employed (Bekker, 1959, 1960). Further testing of undiluted serum in a final concentration of 1:6, as performed in our laboratory, is deemed essential. The use of serum in a final dilution of 1:17 *5 as described in the "Manual of Serologic Tests for Syphilis" (U.S. Dept. of Health, 1964) considerably reduces the number of reactive results. This can be demonstrated from our findings on 1,260 syphilitic sera, in which 173 (13-7 per cent.) showed a reactive RPCF test only with undiluted serum (final dilution 1:6) and 344 (19 * 7 per cent.) exhibited a serum titre of 1:2 (final dilution 1:12). It can be readily seen that syphilis would not have been detected in 33 *4 per cent. of the sera tested had procedures been employed in which the initial final dilution was greater than 1:12. The observation by one of us that a non-reactive test on undiluted syphilitic sera may be due to a prezone or related phenomenon (Bekker, 1962) stresses the importance of testing each serum