Introduction A magnetic particle based assay was developed for the detection of nontreponemal anti-cardiolipin antibodies in sera of suspected syphilis cases. The presence of this group of antibodies in combination with a reactive treponemal test indicates active syphilis. In this study, we aimed to overcome technical difficulties with attaching cardiolipin to solid support. The newly developed assay potentially offers advantages of better result interpretation, accuracy, and minimum equipment need compared to traditional nontreponemal tests in diagnosing syphilis.
Methods To develop the nontreponemal magnetic agglutination assay (NT-MAA), cardiolipin antigen was modified first through a chemical oxidation process. The oxidised antigen was later covalently linked to magnetic particles. To test the beads, serum samples were mixed with cardiolipin-magnetic particle complex, and incubated in round bottom well microplates. The test was interpreted as reactive when agglutination was observed. Non-reactive sample demonstrated a “button” in the centre of a microwell. The NT-MAA was evaluated using a panel of previously characterised human sera (n=80) and results were compared to rapid plasma reagin (RPR, ASI) and Treponema pallidum particle agglutination tests (TP-PA, Fujirebio). A true positive sample was defined as being reactive for both RPR and TP-PA, while a true negative as both RPR and TP-PA non-reactive.
Results Out of 80 sera tested, 48 were found true positive and 32 true negative with the reference tests. In comparison, the NT-MAA, demonstrated a sensitivity and specificity of 100% and 96.8%, respectively.
Conclusion Magnetic particle-based assays offer high flexibility because they work with different assay formats. This exploratory study, describes technical advances for development of nontreponemal test (NT-MAA), and also demonstrated an encouraging performance with the studied samples. Additional evaluation with syphilis samples from defined clinical stages of syphilis will help further validate test performance.