A multi-country comparative study of two treponemal tests for the serodiagnosis of Syphilis amongst Men Who Have Sex with Men (MSM): Chemo-luminescent assay vs Treponema pallidum passive particle agglutination assay
Authors: Gios Lorenzo , Mirandola Massimo , Cordioli Maddalena , Zorzi Antonella , Wlazly Dominika , Hassan-Ibrahim Mohammed Osman , Padovese Valeska , Darmanin Anabel , Peeling Rosanna W , Unemo Magnus , Sherriff Nigel , Blondeel Karel , Toskin Igor , Vera Jaime 
Published: April 2023

Introduction:
International guidelines recommend routine screening for syphilis (aetiological agent: Treponema pallidum subspecies pallidum) amongst key populations and vulnerable populations using tests detecting treponemal and non-treponemal antibodies. Whilst treponemal tests have high sensitivities and specificities, they differ regarding subjective or objective interpretation, throughput and workload. Chemiluminescence immunoassays (CLIAs) are cost- and time-effective automated methods for detecting treponemal antibodies. The Treponema pallidum passive particle agglutination assay (TPPA) has been considered the “gold standard” treponemal assay, however, this includes a highly manual procedure, low throughput and subjective interpretation. The present multi-country study evaluated the ADVIA Centaur® Syphilis CLIA assay compared to the reference SERODIA-TP·PA® for the serodiagnosis of syphilis amongst men who have sex with men (MSM).
Method:
1,485 MSM were enrolled in Brighton (UK), Malta, and Verona (Italy) as part of a larger WHO multi-country and multi-site ProSPeRo study. Ethical approval was obtained. Serum was tested with the ADVIA Centaur® Syphilis CLIA assay (Siemens) and SERODIA-TP·PA® (Fujirebio), in accordance with the manufacturers’ instructions, for a first round of validation. A second round of validation was carried out for discrepant results that were additionally tested with both Western Blot (Westernblot EUROIMMUN ®) and an Immunoblot (INNO-LIA, Fujirebio Diagnostics).
Sensitivity, specificity, positive and negative predictive value (PPV and NPV), likelihood ratios (positive/negative), and the Diagnostic Odds Ratio (DOR)/pre-post-test probability (Fagan's nomogram) were calculated.
Results:
Out of 1,485 eligible samples analysed in the first phase, the SERODIA-TP·PA® (Fujirebio) identified 360 positive and 1,125 negative cases. The ADVIA Centaur® Syphilis CLIA assay (Siemens) identified 366 positives, missclassifying one TPPA-positive sample. In the second phase, the ADVIA CLIA resulted in 1 false negative and 4 false positive results. Considering the syphilis study prevalence of 24% (95% CI: 22-26.7), the sensitivity and specificity of the CLIA in the second phase was 99.7% (95% CI: 97.9-100) and 99.5% (95% CI: 98.8-99.8), respectively. The area under the ROC curve was 0.997 (95% CI: 0.994-1). The PPV and NPV was 98.9% (95% CI: 97.2-99.7) and 99.9% (95% CI: 99.5-100), respectively.
Conclusions:
The ADVIA Centaur® Syphilis CLIA assay showed similar performance compared to the SERODIA-TP·PA®. Considering the study is based on QUADAS principles and with a homogeneous population, results are also likely to be generalisable. The automated CLIA treponemal assay confirmed to be accurate and appropriate for routine initial syphilis screening, i.e. when the reverse testing algorithm is applied.
Ethics:
Prior to enrolment, the research protocol was independently peer reviewed and approved by both the WHO and the site Research Ethics Committee. Participants were enrolled after signing informed consent.