Introduction We aimed to compare two commercial enzyme immunoassays (EIA) for the detection of IgG and IgM anti- Treponema pallidum (Tp) antibodies.

Methods Serum samples were collected in the context of a larger study looking at diagnostic biomarkers for syphilis. Patients with syphilis diagnosed by a treponemal and a non-treponemal assay, were followed for up to two years after treatment. Specimens collected at visit of diagnosis (B), and after three (M3) and six months (M6) of treatment were tested by EIAs detecting anti-Tp IgG and IgM from the manufacturers Euroimmun (EU) and Mikrogen (MI).

Results We tested 338 samples collected from 119 new syphilis cases (23 primary (P), 49 secondary (S), 31 early latent (EL), 16 late latent (LL)) and 30 uninfected controls. A total of 40 participants contributed to 1 sample, 29 to 2 samples and 80 to 3 samples. The controls contributed only to the samples collected at B. Overall 147, 86 and 105 samples were obtained at B, M3 and M6, respectively. The IgM assays were in agreement for 78,1% of samples; it varied according to the syphilis stage: P: 82,1%; S: 72,5%; EL: 80,8%; LL: 72,1% and decreased from B to M3: B: 84,4%; M3: 73,3%; M6: 73,3%. More samples tested positive with the MI (149) versus the EU (100) (p<0.001). EU tested all control samples IgM negative, MI reported 1 positive and 1 borderline. The agreement of both IgG assays was 97,4%; it increased with the stage of infection: P: 91,1%; S: 97,7%; EL: 100%; LL: 100%, and over time: B: 95,2%: M3: 98,8%; M6: 99,0%. More samples tested positive with the EU (305) versus the MI (300) assay. EU reported all control samples IgG negative, MI detected 1 borderline sample.

Conclusion A good but not perfect agreement was observed for the EIAs detecting IgM. The agreement was highest in primary syphilis and lowest in late latent cases, and decreased over time of treatment. The MI IgM assay reported significantly more positive samples. Overall, we found a good agreement for the EIAs detecting IgG. Albeit that it was somewhat lower for primary syphilis and at baseline.