Screening patients for syphilis serology using a "treponemal assay-first" approach presents unique challenges, particularly when applied to low-prevalence populations. The use of a screening algorithm that incorporates semiquantitative values from treponemal antibody test results can help to identify potential false-positive results while requiring a minimum of repeat testing.Syphilis is a sexually transmitted disease caused by infection with the spirochete Treponema pallidum. Because of an inability to routinely culture the infectious agent, diagnosis of syphilis infection is primarily done by a combination of clinical presentation and serology. This serologic testing can be broadly divided into two types of assays: treponemal tests that test for antibodies directed against T. pallidum and nontreponemal tests that measure anticardiolipin antibodies produced during active infection.In recent years, many laboratories have shifted to screening patients with a treponemal assay and then reflexing positive samples to nontreponemal testing, a practice which reverses the historical approach. There is ongoing debate in the literature about the relative merits of the algorithms. While a portion of the discussion focuses on the clinical relevance of identifying treponemal antibodies in asymptomatic patients and the cost-effectiveness of different screening algorithms (4,8,9), an additional concern is the analytical performance of treponemal tests available on the market (10). Although initial studies suggested that the rate of analytical false-positive results was relatively low when using treponemal antibody tests for screening (3), a more recent survey identified a higher frequency of unconfirmed positive results (2). However, these results are somewhat difficult to interpret, as several different combinations of screening and confirmatory tests were used. While many studies have compared the performance characteristics of various marketed treponemal assays (1, 5, 7) one limitation is that treponemal serology results are usually considered to represent a binary variable (i.e., "reactive" versus "nonreactive") rather than a continuous one. To address this, we examined whether semiquantitative results provide additional information relevant to determining a patient's serologic status.Samples were analyzed for the presence of treponemal antibodies by the use of two immunoassays: the Bioplex 2200 syphilis IgG assay (SYPHG) (Bio-Rad Laboratories, Hercules, CA) and the Trep-Sure assay (Phoenix BioTech Corp., Oakville, Ontario, Canada). The Bioplex SYPHG assay is a bead-based multiplex immunoassay that uses recombinant treponemal antigens (Tp15, Tp17, and Tp47) as the capture reagent, followed by detection with a murine anti-human IgGphycoerythrin (PE) conjugate (6). Results are expressed as an "antibody index" (AI), which is an arbitrary unit related to the ratio of sample signal to calibrator-defined cutoffs. Trep-Sure is a microplate-based enzyme immunoassay (EIA) that also uses recombinant treponemal antigens (in a proprie...

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