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INTRODUCTION: Although the rapid plasma reagin (RPR) test is used to determine treatment efficacy for syphilis, animal studies show that it decreases gradually after an initial increase even without treatment. Pre-treatment changes in RPR titer in humans and its relationship with post-treatment changes in RPR titer are not well known. METHODS: We retrospectively analyzed the clinical records of syphilitic patients who underwent automated RPR (Mediace) testing twice before treatment (i.e., at diagnosis and treatment initiation) within 1-3 months at an HIV/AIDS referral hospital in Japan between 2006 and 2018. The RPR values were expressed as the ratio to the value at treatment initiation. The mean monthly relative change in the RPR after treatment was calculated on the log2 scale for each patient and analyzed by multivariable linear regression. RESULTS: Sixty-eight patients were identified. The median age was 45 (interquartile range [IQR], 38-50), 98.5% (67/68) were men, and 97.1% (66/68) had HIV. The median RPR titer ratio at treatment initiation/diagnosis was 0.87 (IQR, 0.48-1.30). The RPR titer decreased more than twofold in 26.5% (18/68) and more than fourfold in 10.3% (7/68) before treatment. In the multivariable analysis, higher age (predicted monthly RPR relative change on the log2 scale 0.23/10 years [95% confidence interval [CI], 0.090-0.37]), history of syphilis (0.36 [95% CI, 0.07-0.65]), and a lower ratio of RPR at treatment initiation/diagnosis (-0.52/every 10-fold increase [95% CI, -0.81 to -0.22]) were associated with a slower RPR decrease after treatment. CONCLUSIONS: In a mostly HIV patient population, RPR titer can show more than four-fold spontaneous increase or decrease within 1-3 months. Pre-treatment spontaneous decrease of RPR titer was associated with a slower decrease in post-treatment RPR titer.

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PLoS One

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