The invention relates to a method for detecting antibodies against the TpN17 antigen of Treponema pallidum in an isolated sample wherein a peptide sequence of Vibrio cholerae lipoprotein 15 (VcLp15) or a partial sequence thereof is used as a reagent for reduction of interference, i.e. for minimizing false positive results. In addition the invention relates to fusion polypeptides comprising a VcLp15 peptide sequence and a chaperone, to their use as an additive in an immunoassay for said reduction of interferences and for minimizing false positive results and to a reagent kit for detecting antibodies against Treponema pallidum antigens in an isolated sample comprising a TpN17 antigen and said VcLp15-chaperone fusion polypeptide.