A retrospective study on nested PCR detection of syphilis treponemes in clinical samples: PCR detection contributes to the diagnosis of syphilis in patients with seronegative and serodiscrepant results.

Eliška Vrbová,David Šmajs,Vladimír Vašků,Lenka Mikalová,Daniela Vaňousová,Petra Pospíšilová,Radim Strnadel,Vladana Woznicová,Linda Grillová,Přemysl Procházka,Filip Rob,Ivana Kuklová,Miluše Kreidlová,Alena Krchňáková,Martina Kojanová,Eliška Dastychová,Monika Dvořáková Heroldová,Hana Zákoucká,Simon Clegg

doi:10.1371/journal.pone.0237949

Eliška Vrbová, David Šmajs + Show 17 more

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https://doi.org/10.1371/journal.pone.0237949

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Abstract

Syphilis, caused by Treponema pallidum ssp. pallidum (TPA), is a persisting global health problem. Although syphilis diagnostics relies mainly on serology, serological tests have some limitations, and it is recommended that the final diagnosis be supported by additional tests. The purpose of this study was to analyze the relationship between serology and PCR in syphilis diagnostics. From the year 2004 to May 2019, a total of 941 samples were taken from 833 patients suspected of having syphilis, in Czech Republic. In all these samples, both nested PCR detection of TPA and serology testing were performed. Of the 941 samples, 126 were seronegative, 651 were seropositive, and 164 were serodiscrepant. Among seronegative samples (n = 126), 11 were PCR-positive (8.7%). Among seropositive samples (n = 651; i.e., samples positive for both non-treponemal and treponemal serology tests), 368 samples were PCR-positive (56.5%). The remaining 164 serodiscrepant samples included RPR negative and treponemal serological test-positive samples (n = 154) and a set of 10 RPR-positive samples negative in treponemal serological tests. While the first group revealed 73 PCR-positive samples (47.4%), the second revealed 5 PCR positive samples (50.0%). PCR detection rates were highest in primary syphilis, with lower rates in the secondary and undetermined syphilis stages. As shown here, the nested PCR can improve diagnostics of syphilis, especially in seronegative patients and in patients with discrepant serology.

Highlights

Syphilis is a multi-stage venereal disease, which is caused by the Treponema pallidum subsp. pallidum (TPA) bacterium
In general, a high specificity and sensitivity, they have several limitations: (1) there is reduced sensitivity in the early and late stages of syphilis, (2) there is a risk of false-positive reactions, which can be caused by other acute or chronic infections, and (3) the non-treponemal tests are susceptible to false-negative results due to the prozone effect [1, 3, 4]
Sensitivity, specificity, positive and negative predictive values, were 72.3%, 100%, 100%, 41.5% for PCR and 89.3%, 100%, 100%, 69.3% for serology, respectively. These results are comparable with the results reported by Noda et al [15] with slightly lower sensitivity and negative predictive value of PCR and slightly higher sensitivity and negative predictive value of serology

Summary

Introduction

Syphilis is a multi-stage venereal disease, which is caused by the Treponema pallidum subsp. pallidum (TPA) bacterium. Non-treponemal tests include the Rapid Plasma Reagin (RPR) test and the Venereal Disease Research Laboratory (VDRL) test. These tests detect antibodies against cardiolipin, high titers of which are found in ongoing infections. Treponemal tests include the Fluorescent Treponemal Antibody Absorption Test (FTA-Abs), T. pallidum Hemagglutination Assay (TPHA), and T. pallidum Particle Agglutination Assay (TPPA). In general, a high specificity and sensitivity, they have several limitations: (1) there is reduced sensitivity in the early and late stages of syphilis, (2) there is a risk of false-positive reactions, which can be caused by other acute or chronic infections, and (3) the non-treponemal tests are susceptible to false-negative results due to the prozone effect [1, 3, 4]. Previous studies have shown that the best samples for treponemal DNA detection come from swabs taken from syphilitic ulcers, rather than whole blood samples [7, 8]

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