Comparison of 17 serological treponemal and nontreponemal assays for syphilis: A retrospective cohort study

OBJECTIVES: Rapid plasma reagin (RPR) and Treponema pallidum (TP) antibody test kits are often used to diagnose syphilis, although the relationship between their measured values is unclear. We aimed to reveal the relevance of these kits’ results. DESIGN AND METHODS: In all, 143 sera from 110 patients were tested using 12 TP kits and 5 RPR kits and the results compared. RESULTS: The specificity and sensitivity of RPR kits were 81–96% and 95–100%, respectively. The correlation coefficients (0.849–0.934) considerably differed between the manual RPR card test and latex agglutination (LA) assay kits. The following sensitivities were obtained: 82–91% for TP fluorescent treponemal antibody absorption assay (FTA-ABS), TP hemagglutination assay (HA), and TP particle agglutination assay (PA); 94–95% for TP LAs; and 92–100% for chemiluminescent immunoassay (CLIA), chemiluminescent enzyme immunoassay (CLEIA), and immunochromatography assay (IC). Correlation coefficients between TP kits were 0.753–0.974, and the measured values varied. Changes in RPR and quantifiable TP kits were the same for patients with reinfected syphilis and with syphilis under treatment. CONCLUSIONS: RPR tests had lower specificity than TP antibody tests. RPR card test and RPR LAs had similar specificity and sensitivity, but their measured values were different. RPR should be measured using automatic RPR LA without setting the upper limit of the reported value. RPR LA should also be standardized. The sensitivity of TP antibody was better in CLIA, CLEIA, and IC than in FTA-ABS, HA, PA, and LA. Therefore, TP antibody kits should be standardized and quantified.