Relationship between DiaSorin Liaison Treponema pallidum antibody indices and confirmatory assay results in the...

Published studies show that >99% of sera reactive in the reverse syphilis testing algorithm (RSTA) screening assay with an index above an assay-specific threshold confirm as reactive, with either a rapid plasma reagin-reactive (RPR^R) or RPR-nonreactive/Treponema pallidum particle agglutination-reactive (TPPA^R) result. However, the relationship between screen indices and confirmatory patterns has not been characterized. We thus assessed confirmatory testing results for 577 sera submitted for RSTA testing and a screen-reactive result in the DiaSorin Liaison assay. The median screen index was significantly higher for RPR^R samples than TPPA^R samples (55.6 versus 10.4), and the proportion with indices >28.3 (median for all 577 samples) was significantly higher for RPR^R versus TPPA^R samples (82% versus 26%). However, RPR^R titers did not significantly correlate with screen indices (R² = 0.02). These findings demonstrate a significant relationship between RSTA screen indices and confirmatory assay results. The clinical utility of this relationship requires further study.