Repeat syphilis is playing an increasing role in syphilis transmission in several populations. The assessment of repeat syphilis and response to treatment depends on accurately measuring intraindividual changes in non–treponemal tests. For a 0- to 6-month delta rapid plasma reagin (RPR) to be determined by routine individual RPR testing, samples are tested 6 months apart with differences in reagent batches, environmental conditions, and observers all leading to measurement errors. We hypothesized that conducting paired RPR testing (simultaneous testing of acute and convalescent samples) would enable a more accurate determination of delta RPR compared with individual testing.
A total of 120 study participants with a new diagnosis of syphilis were followed up at 0, 3, 6, 9, 12, 18, and 24 months, with RPR testing performed via individual testing at each study visit and at any suspected repeat syphilis. Rapid plasma reagin paired testing was performed on samples from 0 and 6 months and at any suspected repeat syphilis.
The quantitative agreement ±1 dilution among paired and individual testing was 97.2%. There was no difference in the proportion with serofast status at 6 months: 21 (19.4%) and 19 (17.6%) according to paired and individual testing, respectively (P = 0.726). There was no statistically significant difference between 0- and 6-month delta RPR as determined by paired and individual testing in predicting seroresponse at 12 months (86.1% and 91.6% agreement with 12-month serofast/nonserofast classification, respectively; P = 0.262).
In our setting, individual testing performed equally well compared with paired testing. Follow-up of syphilis will remain onerous for the patient and the health care provider until new tests that can more accurately assess the response to therapy and repeat syphilis/treatment failure are developed.
A longitudinal study of clients with syphilis found that paired rapid plasma reagin testing (simultaneous testing of acute and convalescent samples) as opposed to individual rapid plasma reagin testing was not useful. There was no difference in the proportion classified as serofast at 6 or 12 months.
From the *HIV/STI Unit, Institute of Tropical Medicine, Antwerp, Belgium; †Division of Infectious Diseases and HIV Medicine, University of Cape Town, Observatory, South Africa; ‡Institute of Tropical Medicine; and §HIV/AIDS and Infectious Diseases Unit; and ¶HIV/STI Reference Laboratory, Institute of Tropical Medicine, Antwerp, Belgium
Acknowledgment: The authors would like to thank the study recruits for their participation and Vicky Cuylaerts and Nikki Foque for performing the laboratory assays.
Disclosure/Conflict of interest: The authors report no conflict of interest.
Funding: This work was part of Project ID 757003 funded by the Flemish Government, Department of Economy, Science and Innovation.
Authors' contributions: C.K. conceptualized the study. C.K. was responsible for the acquisition, analysis, and interpretation of data. C.K., K.K.O., T.C., L.L., and M.v.E. played a role in writing, editing, and approving the final version.
Correspondence: Chris Kenyon, MD, PhD, MPH, Division of Infectious Diseases and HIV Medicine, University of Cape Town, Anzio Road, Observatory 7700, South Africa. E-mail: firstname.lastname@example.org.
Received for publication May 12, 2017, and accepted June 29, 2017.