Characteristics of patients with primary and late latent syphilis patients who were initially non-reactive to the rapid plasma reagin test.

We aimed at determining the characteristics of patients with primary and late latent syphilis who were non-reactive on initial screening by rapid plasma reagin (RPR) but reactive by treponemal tests. We collected the RPR test results of all primary and late latent syphilis patients in our hospital from December 2000 to March 2012. The characteristics of syphilis patients who were non-reactive by RPR testing were compared to those of reactive patients. Multiple logistic regression was used to identify factors associated with non-reactive RPR results. Among primary syphilis patients, 37 (16.5%) were non-reactive on initial RPR and were compared with the 187 reactive cases. Age >35 years was an independent factor associated with a non-reactive result in primary cases (odds ratio [OR], 95% confidence intervals [CI] = 3.55 [1.39-9.07]). Of the late latent patients, 61 (8.8%) were non-reactive by RPR and 636 were reactive. Age >34 years was also an independent factor associated with a non-reactive result in late latent cases (OR [95% CI] = 4.30 [2.28-8.12]). This study suggests that RPR testing alone is insufficient to diagnose primary and late latent infections, especially in middle-aged and elderly individuals. Syphilis detection was lower for patients with primary syphilis than for those with late latent syphilis based on the results of the RPR.