Background Biologic false positive (BFP) non-treponemal test results, defined as specimens with reactive non-treponemal (NT) and non-reactive treponemal test results are received and processed by United States public health surveillance programs. Non-treponemal BFPs can be attributed to a variety of infectious and non-infectious diseases. However, little is known about the distribution of BFP NT titers. We describe the frequency, NT titer distribution, and descriptive characteristics of people with BFP NT results reported in Florida and New York City (NYC).
Methods Reactive NTs and BFPs (reactive NT test and ≥ 1 non-reactive treponemal tests from a person with no past or current reactive treponemal test) were extracted from Florida’s and NYC’s sexually transmitted diseases surveillance systems for 2013–2017. Results were de-duplicated by specimen collection date, test type, and titer value. For individuals, BFPs were stratified by site, patient characteristics, and highest reported titer during study period.
Results A total of 526,540 reactive NTs were reported. Among these were 57,850 BFPs (28,183 Florida and 29,397 NYC), 11% of all reactive NTs, from 39,920 individuals (19,313 Florida and 20,607 NYC). Titers of 1:1 accounted for 55% (n=31,580) of all BFPs, but 5,250 (9%) were ≥ 1:8, including 654 (1.1%) ≥ 1:32. Persons with BFP were most often women 68% (n=27,161/39,920). Individuals 40+ years were at increased odds (OR 2.12; 95% C.I. 1.78–2.55) of having a high titer BFP (≥ 1:32 titer).
Conclusion Syphilis BFPs (non-cases) account for a substantial number of reported tests that require processing. Some countries classify titers ≥1:8 as presumptive syphilis without treponemal testing, thus may over-count cases. Areas requiring both treponemal and NT tests for syphilis case reporting may benefit from requiring laboratories to report negative treponemal tests when they are associated with reactive NT tests. Review of patient histories might identify underlying conditions that contribute to high-titer BFP results.
Disclosure No significant relationships.
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