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Original research
Prevalence and clinical characteristics of patients with biologically false-positive reactions with serological syphilis testing in contemporary practice: 10-year experience at a tertiary academic hospital
  1. Yuri Ishihara1,
  2. Koh Okamoto2,
  3. Hironori Shimosaka1,
  4. Yoshikazu Ono1,
  5. Yoshiaki Kanno2,
  6. Mahoko Ikeda2,
  7. Sohei Harada2,3,
  8. Makoto Kurano1,
  9. Shu Okugawa2,
  10. Kyoji Moriya2,3,
  11. Yutaka Yatomi1
  1. 1Department of Laboratory Medicine, The University of Tokyo Hospital, Tokyo, Japan
  2. 2Department of Infectious Diseases, The University of Tokyo Hospital, Tokyo, Japan
  3. 3Department of Infection Control and Prevention, The University of Tokyo Hospital, Tokyo, Japan
  1. Correspondence to Dr Koh Okamoto, Department of Infectious Diseases, The University of Tokyo Hospital, Tokyo, Japan; kohokamoto1980{at}


Objectives Biologically false positive (BFP) reactions are well described in early literature. However, only a few recent reports described the incidence and clinical characteristics of patients with BFP reactions. We reviewed the serological test results of patients tested for syphilis in our hospital in the past decade and described the clinical characteristics of patients with BFP reactions.

Methods This is a retrospective study of patients tested for syphilis in a tertiary academic hospital. All serological results were retrieved from the clinical laboratory database. We calculated the incidence of BFP reactions. Clinical characteristics and laboratory data of patients with BFP reactions were reviewed manually.

Results Among 94 462 subjects, 588 patients had BFP reactions (0.62%). Most BFP reactions were observed in patients aged over 60 years, with a history of malignancy and autoimmune diseases. Eighty-five per cent of patients had low rapid plasma reagin (RPR) titre (≤1:4), but two patients had extremely high RPR titre (≥1:256). BFP reactions were more likely to persist beyond 6 months among patients with RPR titre of ≥1:8. There was no statistically significant correlation between RPR titre and total protein albumin gap, surrogate of immunoglobulin levels among patients with BFP reactions.

Conclusion There was a low incidence of BFP reactions in the last decade. A minority of BFP reactions had high non-treponemal antibody titre and persisted longer than 6 months. In the era of re-emergence of syphilis, this information could help clinicians interpret the results of well-established diagnostic tests for syphilis.

  • syphilis
  • serology
  • laboratory methods

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  • Handling editor Jason J Ong

  • YI and KO contributed equally.

  • Contributors Conception and design of study, acquisition of data, drafting of the manuscript: YI and KO; analysis and/or interpretation of data: YI, KO, YK, MI, SH and SO; revision of the manuscript critically for important intellectual content: YK, MI, SH, MK and SO; approval of the version of the manuscript to be published: YI, KO, HS, YO, YK, MI, SH, MK, SO, KM and YY.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval The present study was approved by the ethics board of the University of Tokyo, School of Medicine (number 3683).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.

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