Screening for syphilis with the treponemal immunoassay: analysis of discordant serology results and implications for clinical management.

BACKGROUND: Screening for syphilis with treponemal chemiluminescence immunoassays (CIA) identifies patients with discordant serology who are not identified with traditional screening methods (eg, CIA-positive, rapid plasma regain (RPR)-negative). We sought to describe the clinical characteristics and management of patients with discordant syphilis serology.
METHODS: From August 2007-October 2007, patients with CIA-positive, RPR-negative serology were tested with the Treponema pallidum particle agglutination assay (TP-PA) at Kaiser Permanente Northern California. Clinical and demographic characteristics, prior syphilis history and CIA index values were compared for CIA-positive, RPR-negative patients according to TP-PA status.
RESULTS: Of 21,623 assays, 439 (2%) were CIA-positive and 255/439 (58%) were RPR-negative; subsequently, 184 (72%) were TP-PA-positive and 71 (28%) were TP-PA--negative. TP-PA--positive patients were more likely to be male, HIV-positive, homosexual, previously treated for syphilis (57% versus 9%), with higher median CIA index values (9.8 versus 1.6) (all P < .0001). After repeat testing, 7/31 (23%) CIA-positive, RPR-negative, TP-PA--negative patients seroreverted to CIA-negative.
CONCLUSIONS: TP-PA results in conjunction with clinical/behavioral assessment helped guide the management of patients with CIA-positive, RPR-negative serology. TP-PA-positive patients were both highly likely to have prior syphilis and major epidemiologic risk factors for syphilis. CIA-positive, RPR-negative, TP-PA-negative serology may represent a false-positive CIA in low-prevalence populations.