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Background: The laboratory diagnosis of syphilis relies on serological tests. The Illinois Department of Public Health (IDPH) Laboratory replaced the rapid plasma reagin (RPR) with the treponemal enzyme immunoassay (EIA) (Trep-SureTM) as the initial syphilis screening test in February 2008. Reactive syphilis EIA tests are confirmed by algorithm with the RPR and fluorescent treponemal antibody absorption (FTA-ABS) tests.
Objectives: To compare syphilis EIA test results with RPR and FTA-ABS results to identify and interpret discordant outcomes.
Methods: We analyzed results of syphilis serological tests of patients presenting to five Chicago Department of Public Health (CDPH) STI Clinics from 2008-2009. Discordant results were compared with surveillance data to obtain syphilis history and treatment information.
Results: Between February 2008 and March 2009, 29,436 specimens were submitted to IDPH laboratory for syphilis testing from the five Chicago municipal STI Specialty Clinics. Discordant results between the EIA and FTA-ABS tests were found in 1.06% (312/29,436) of specimens; 48% (149/312) of discordant results were reactive by EIA and nonreactive by RPR and FTA-ABS, and 39% (123/312) of discordant results were reactive by EIA, nonreactive by RPR, and equivocal by FTA-ABS. Nonreactive EIA and reactive FTA-ABS results were identified in 13% (40/312) of discordant results. Data were further stratified by RPR test results, and presented in the table below. EIA=P RPR=N FTA=E #(%) EIA=P RPR=N FTA=N #(%) EIA=N RPR=R FTA=R #(%) EIA=N RPR=N/A FTA=R #(%) Total #(%) Total 123(39%) 149(48%) 5(1.6%) 35(11.2%) 312
Conclusions: Interpretation of discordant syphilis test results is challenging and requires information on syphilis history and clinical symptomatology. Despite the high sensitivity of the EIA screening test, syphilis cases with nonreactive EIA and reactive RPR and FTA-ABS discordant results might be missed.
Implications for Programs, Policy, and/or Research: With the advent of syphilis EIA tests, clinicians should remain vigilant and interpret test results in correlation with clinical data.