Abstract
Objectives: Immunoassays suffer from false positives due to interference from non-specific antibodies, regardless of the target antigen or antibody under test. This research investigated the possibility that two human antibodies; anti-Gal and anti-Forsmann (Fs), were a cause of false positives in a syphilis immunoassay. A further aim was to establish the minimum concentration of synthetic antigens able to block activity of anti-Gal and anti-Fs in human plasma samples. Methods: Twenty plasma donations which had tested false positive for syphilis in a blood donor screening assay were tested in a rapid immunoassay for syphilis. Subsequently, anti-Gal and anti-Fs activity was blocked in each sample with synthetic [alpha]Gal and Fs antigens. Samples which tested positive in the rapid test were re-tested with plasma subjected to blocking. All 20 samples were studied for levels of anti-Gal and anti-Fs by agglutination with kodecytes. The minimum concentration of synthetic antigens able to block anti-Gal and anti-Fs was established using different concentrations of polyacrylamide conjugates of the corresponding antigens, with subsequent antibody reactivity tested with antigen-specific kodecytes. Results: Three of the 20 samples which tested false positive in the blood donor screening assay tested positive with the rapid test for syphilis antibodies. None of the three samples yielded a negative rapid test result when blocked with synthetic [alpha]Gal or Fs antigens. Levels of anti-Gal and anti-Fs showed no correlation with results of the rapid test for syphilis antibodies. The minimum concentration of synthetic antigens required to neutralise (block) antibody activity was 250 [micro]M for anti-Gal, and 125 [micro]M for anti-Fs. Conclusions: Anti-Gal and anti-Fs are unlikely to the be a cause of false positive reactions in immunoassays. Key words: Immunoassay, [alpha]Gal, Forssman, kodecytes.