日本性感染症学会誌 Journal of Japanese Society for Sexually Transmitted Infections

A “RPR negative and treponemal antibody positive” serum may result from two different stages of syphilis: primary syphilis or cured syphilis. Herein, we investigated certain seroanalytic methods to differentiate these two stages. 1) Absorptance: it is calculated using treponemal antibody titers with or without the treponemal antibody absorption reagent (treponemal antigen). No significant difference was observed between the treponemal antibody titers of primary and cured syphilis sera. However, the absorptance values between the two groups differed significantly. 2) Absorptance ratio: this is the ratio of an absorptance value with a quarter dose of the absorption reagent to that with the usual dose. Absorptance ratios of primary syphilis sera were lower than those of cured syphilis sera. Using 0.8 as a cutoff value for this ratio, we differentiated untreated primary syphilis from cured syphilis with 90% sensitivity and 100% specificity. Our results indicate that these seroanalytic methods can effectively differentiate untreated primary syphilis from cured syphilis and may serve as a type of avidity test.