Are nontreponemal tests suitable for monitoring syphilis treatment efficacy? Evidence from rabbit infection models

ObjectivesWe aimed to characterize kinetics of non-treponamal antibody titres during the natural course of syphilis and explore their roles in monitoring syphilis treatment efficacy.MethodsSixty New Zealand white male rabbits were challenged with Nichols or Amoy Treponema pallidum strains, and the rapid plasma reagin (RPR) test was performed to quantify non-treponemal antibody titres during the infection course. Viable T. pallidum in the challenged rabbits was assessed with rabbit infectivity tests.ResultsThe RPR titres of the Nichols or Amoy strain between no benzathine penicillin G (BPG) and BPG treatment subgroups displayed a similar trend: first ascending and then descending. Compared with baseline, the proportions of fourfold decline in RPR titres in the Nichols or Amoy group presented a similar result on days 30, 60 and 180 between the no BPG and BPG treatment subgroups (0%, 0/5; 80%, 4/5; 100%, 5/5; vs. 0%, 0/5; 80%, 4/5; 100%, 5/5; p 0.999; 0%, 0/5; 80%, 4/5; 80%, 4/5; vs. 40%, 2/5; 100%, 5/5; 100%, 5/5; p 0.098, respectively). Compared with the maximum baseline titre, the proportion of fourfold decline in PRR titre also showed a similar result in the two groups on days 30, 60 and 180 between the no BPG and the BPG treatment subgroups (0%, 0/5; 100%, 5/5; 100%, 5/5, vs. 40%, 2/5; 100%, 5/5; 100%, 5/5; p 0.129; 0%, 0/5; 100%, 5/5; 100%, 5/5, vs. 80%, 4/5; 100%, 5/5; 100%, 5/5; p 0.091, respectively. Moreover, regardless of whether the RPR titres presented a fourfold decline, viable T. pallidum could be detected in untreated rabbits' lymph nodes at 30, 60 and 180 days post infection, while viable T. pallidum was not detected in any of the treated rabbits' lymph nodes.ConclusionsThe RPR titre increased and then decreased (even became negative) during the natural course of syphilis, similar to that seen after BPG treatment. The RPR tetre is thus a questionable indicator of syphilis treatment efficacy.     

Retrospective comparison between non-treponemal and treponemal tests for screening of blood donors for syphilis and their correlation with donor history in a tertiary care teaching hospital

Background and objectivesTreponemal tests provide advantage of better detection during early, late and latent stages of syphilis with equal or higher sensitivity & specificity in comparison to non-treponemal tests. The objective of the present study was to analyse the level of concordance between treponemal and non-treponemal tests for donor screening and to correlate them with donor history.Materials and methodsRetrospective analysis of syphilis screening by treponemal (Chemiluminescence & TPHA) and non-treponemal tests (RPR) was done and donor history for high-risk behaviour and factors associated with false positivity were collected from post-donation counselling and collected data was coded and analysed.ResultsAmongst the 12,000 donors screened, reactivity rate by RPR, TPHA and Chemiluminescence was 0.45%, 0.8% and 1.17% respectively. There was discordance of 62% and 32% for reactive results by RPR and TPHA respectively when compared with Chemiluminescence. History of high-risk behaviour was present in ∼ 50% and 15% of donors with discordant results by RPR and TPHA respectively. Of 34 donors who were reactive only by Chemiluminescence and were followed up, 15% had history of high-risk behaviour and 56% had factors associated with false reactivity.ConclusionTreponemal tests showed high syphilis reactivity amongst blood donors as compared to non-treponemal tests most likely due to their ability to detect early, late and latent syphilis cases. This may confer added transfusion safety in centres dependent on replacement donors without NAT testing by identifying donors with high-risk history with negligible increase in discard rate due to false reactivity.