Diagnostic Properties of Different Serological Methods for Syphilis Testing in Brazil

Abstract

Background/Objectives: Syphilis remains a significant public health challenge worldwide. Accurate and efficient diagnostic tools are essential to controlling the spread of the disease. Current diagnostic approaches primarily rely on serologic treponemal tests (TTs) and nontreponemal tests (NTTs). The aim of this study was to evaluate the diagnostic properties of various serological methods for syphilis diagnosis. Methods: Samples were collected from participants of the Health, Information, and Sexually Transmitted Infection Monitoring (SIM study) between March 2020 and May 2023, using convenience sampling at a mobile health unit in Porto Alegre, Brazil. A total of 250 individuals were tested using the point-of-care (POC) lateral flow treponemal test, Venereal Disease Research Laboratory (VDRL) test, Rapid Plasma Reagin (RPR) test, Enzyme-Linked Immunosorbent Assay (ELISA), and Treponema pallidum hemagglutination assay (TPHA). Of these, 125 participants tested positive for syphilis in the POC screening. Diagnostic properties such as sensitivity, specificity, and predictive values were assessed for the POC test, ELISA, and VDRL test. The TPHA was used as the reference standard for the TT, and the RPR test as the reference standard for the NTT. Results: Among individuals with positive POC test results, 97.6% (122/125) were also positive by the ELISA, and 85.6% (107/125) were positive by the TPHA. Additionally, 48.0% (60/125) and 42.4% (53/125) tested positive by the VDRL and RPR tests, respectively. Using the TPHA as a reference, TT tests showed sensitivities of 97-98% and specificities of 93-95% for detecting anti-Treponema pallidum antibodies using the ELISA and POC test, respectively. For the NTT, the VDRL test demonstrated a sensitivity of 98% and a specificity of 95% compared to the RPR test. The kappa coefficients were 0.85 for the POC test vs. the TPHA, 0.81 for the ELISA vs. the TPHA, and 0.89 for the VDRL vs. the RPR tests, indicating substantial agreement. Conclusions: This study highlights a good diagnostic performance and high agreement levels among the evaluated serological tests for syphilis, reinforcing their utility in clinical and public health settings, as well as epidemiological studies.

Keywords: ELISA; POC; RPR; TPHA; VDRL; serological tests; syphilis diagnosis.

Figure 1

Comparison between nontreponemal test titers (VDRL vs. RPR tests). On the x-axis are the titers of the VDRL test, while the y-axis corresponds to the titers of the RPR test. The circles represent the results identified in both tests, with the size of the circle being directly proportional to the frequency of the results found. Additionally, the intensity of the colors represented in the circles is directly proportional to the frequency of the identified results.

Figure 2

Syphilis positive testing flowchart with reverse algorithm showing POC-positive tests following treponemal and nontreponemal testing. (A) POC test followed by VDRL test and TPHA; (B) POC test followed by VDRL test and ELISA; (C) POC test followed by RPR test and TPHA; (D) POC test followed by RPR test and ELISA.

Figure 2

Syphilis positive testing flowchart with reverse algorithm showing POC-positive tests following treponemal and nontreponemal testing. (A) POC test followed by VDRL test and TPHA; (B) POC test followed by VDRL test and ELISA; (C) POC test followed by RPR test and TPHA; (D) POC test followed by RPR test and ELISA.