Background Automated immunoassays (AI) for detection of T. pallidum antibodies are increasingly used for syphilis screening in the United States. These assays demonstrate fast performance, reduced labour requirements, and high throughput with walk-away capability. Limited data are available about the relative seroreactivity among commercial treponemal assays, especially in low risk populations. Additionally, it is unknown to what extent the AI signal strength values, used to assess reactivity, are associated with non-AI treponemal reactivity. We compared concordance of seroreactivity among 7 treponemal tests and assessed AI signal strength values associated with reactivity.
Methods Previously identified reactive and nonreactive sera (n = 566) were obtained from Kaiser Northern and Southern California regional laboratories. All sera were tested with AIs: BioPlex 2200 Syphilis IgM/IgG (BioRad), treponemal LIAISON (DiaSorin), Advia-Centaur syphilis (Siemens), and non-AIs (INNOLIA syphilis score (INNOGENETICS), TrepSure (Phoenix Biotech), Treponemal Pallidum Particle Agglutination (TP-PA) (Fujirebio), and Fluorescent Treponemal Antibody-Absorption (FTA-ABS) (Zeus Scientific) tests. Reactivity was interpreted according to manufacturers’ instructions.
Results Seroreactivity ranged from 40.5 – 43.9% for AIs, and 33.0–42.2% for non-AIs. In all 7 tests, 30% (167/566) were reactive, and positive agreement among assays was 82.3%. The overall seroreactivity among AIs was 38.9% (220/566) and positive agreement was 92.6%. Minimum signal strength values of 11.72 (Centaur, range: 1.1–45), 4.4 (BioPlex, range: 1.1–8) and 9.4 (Liaison, range: 1.1–70) correlated 100% with TPPA reactivity. The proportion of AI-seroreactive specimens that were also TP-PA reactive were: 86.5% (198/229) for BioPlex, 85.2% (202/237) for ADVIA-Centaur, and 81.6% (200/245) for LIAISON.
Conclusion Although there is some variation in seroreactivity among the 7 tests, there is good correlation. A large proportion of AI tests with a minimal signal-to-cutoff ratio were associated with a positive TP-PA, suggesting that a second treponemal test may not be necessary to confirm AI-reactive, RPR-nonreactive sera.
- Automated tests
- Treponemal test
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