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.