Identification of False-Positive Syphilis Antibody Results Using a Semiquantitative Algorithm

Abstract

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...

Analysis of Bioplex Syphilis IgG Quantitative Results in Different Patient Populations

“…The ability of the AI value of the Bioplex IgG assay to predict a reactive TP-PA result varied by patient cohort and by the accompanying RPR titer ( Table 1). As reported by Yen-Lieberman et al (5), specimens that exhibited reactive RPR titers were more likely to exhibit reactive TP-PA results. However, we evaluated specimens with an RPR titer of 1:1 separately from those with titers of Ն2 because of clinical and epidemiological evidence indicating that an RPR titer of 1:1 is not always sufficient to confirm a reactive IgG result.…”

“…Among the incarcerated and OB/Gyn cohorts, an AI cutoff of 8 provided higher specificity than a cutoff of 6. Yen-Lieberman et al showed that an AI of 6 provided 100% specificity for their study population (5). Even at a cutoff of 8, the specificity of the Bioplex IgG assay in our study failed to reach 100% in the incarcerated cohort for specimens with RPR-nonreactive results or with a titer of 1:1 (often considered an equivocal titer).…”

Point-Counterpoint: It Is Time To Use Treponema-Specific Antibody Screening Tests for Diagnosis of Syphilis

“…When the IgG test is reactive, RPR titer determination and TPPA are performed and reported in the afternoon. Yen-Lieberman et al reported that a positive RPR result was sufficient to verify syphilis (17). However, we found that a weakly reactive RPR titer of 1:1 was not always sufficient to verify syphilis in specimens reactive by IgG MFI, while specimens with RPR titers of Ն1:2 were always confirmed positive by TPPA (9).…”

Correlation of Treponemal Immunoassay Signal Strength Values with Reactivity of Confirmatory Treponemal Testing

“…Several analyses have concluded that treponemal immunoassay signal strength values may be used in lieu of confirmatory testing with a second treponemal test: two studies of a microbead immunoassay (MBIA), one of a CIA, and another of an EIA (5)(6)(7)(8).…”