TP-PA shows excellent sensitivity for adjudicating syphilis

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Treponema pallidum particle agglutination assay (TP-PA) is a better test to adjudicate syphilis results given its high specificity and superior sensitivity, according to a study.

Syphilis is increasing at epidemic rates in the US, especially among men who have sex with men, women of reproductive age, and new-borns. Traditionally, syphilis diagnosis involves use of non-treponemal serology (rapid plasma reagin, Venereal Disease Research Laboratory) directed against lipoidal antigens including lecithin, cardiolipin, and cholesterol. Confirmation of reactive results is performed with a treponemal test (TP-PA).
Although non-treponemal tests are inexpensive and useful for monitoring treatment response, they require significant hands-on time by laboratory personnel.

This study led by corresponding author Ina Park at the STD Control Branch, division of communicable disease control, California Department of Public Health and division of STD Prevention, US Centres for Disease Control and Prevention, Atlanta, compared the sensitivity and specificity of newer automated treponemal tests (enzyme immunoassay [EIA], chemiluminescence immunoassay [CIA], microbead immunoassay [MBIA)] and manual treponemal tests (fluorescent treponemal antibody absorption test [FTA-ABS], TP-PA) in individuals with and without a clinical diagnosis of syphilis.

The findings will help inform the most appropriate second treponemal test selection for patients with initially discordant treponemal and nontreponemal serology, and selection of an automated treponemal test when the reverse sequence algorithm is used for a laboratory diagnosis of syphilis.

A total of 959 participants were included. Cases were characterised as: 1) current clinical diagnosis of syphilis: primary, secondary, early latent, or late latent (n=262); 2) prior treated syphilis (n=294); 3) no evidence of current syphilis no prior history of syphilis and at least 4/7 treponemal tests negative (n=403). Sensitivity and specificity of 7 treponemal assays were calculated, including: 1) ADVIA Centaur (CIA); 2) Bioplex 2200 (MBIA); 3) FTA-ABS; 4) INNO-LIA (line immunoassay); 5) LIAISON CIA; 6) TP-PA; 7) Trep-Sure (EIA).

In examining sensitivity by stage of syphilis, FTA-ABS had the lowest sensitivity for primary syphilis (78.2%) compared to the other immunoassays or TP-PA (94.5%-96.4%) (all P ≤.01). FTA-ABS was also significantly less sensitive for secondary syphilis (P =.007), while all other assays were 100% sensitive for secondary syphilis. All 7 assays demonstrated high sensitivity (95%-100%) for early latent syphilis, and a lower sensitivity (86.8%-98.5%) for late latent disease. TP-PA was significantly less sensitive than TrepSure EIA for late latent disease (P =.009). Overall, the 4 immunoassays routinely used for screening (LIAISON CIA, ADVIA Centaur CIA, TrepSure EIA, Bioplex 2200 MBIA) demonstrated high sensitivity for primary secondary, and early latent syphilis, comparable to traditional manual tests like TP-PA.

Abstract
Background: Treponemal immunoassays are increasingly used for syphilis screening with the reverse sequence algorithm. There are little data describing performance of treponemal immunoassays compared to traditional treponemal tests in patients with and without syphilis.
Methods: We calculated sensitivity and specificity of seven treponemal assays: 1) ADVIA Centaur (chemiluminescence immunoassay-CIA), 2) Bioplex 2200 (microbead immunoassay-MBIA), 3) fluorescent treponemal antibody absorption test (FTA-ABS), 4) INNO-LIA (line immunoassay), 5) LIAISON CIA, 6) TP-PA (Treponema pallidum particle agglutination assay), and 7) Trep-Sure (enzyme immunoassay-EIA), using a reference standard combining clinical diagnosis and serology results. Sera were collected between May 2012-January 2013. Cases were characterized as: 1) current clinical diagnosis of syphilis: primary, secondary, early latent, late latent 2) prior treated syphilis only, 3) no evidence of current syphilis, no prior history of syphilis and at least 4/7 treponemal tests negative.
Results: Among 959 participants, 262 had current syphilis, 294 had prior syphilis, and 403 did not have syphilis. FTA-ABS was less sensitive for primary syphilis [78.2% (65.0-88.2%)], than the immunoassays or TP-PA (94.5-96.4%) (all p≤0.01). All immunoassays were 100% sensitive for secondary syphilis, 95.2-100% sensitive for early latent disease, and 86.8-98.5% sensitive in late latent disease. TP-PA had 100% specificity (99.0-100%).
Conclusion
Treponemal immunoassays demonstrated excellent sensitivity for secondary, early latent, and seropositive primary syphilis. Sensitivity of FTA-ABS in primary syphilis was poor compared to the immunoassays and TP-PA. Given its high specificity and superior sensitivity, TP-PA is a better test to adjudicate discordant results with the reverse sequence algorithm than the FTA-ABS.

Authors
Ina U Park, Yetunde F Fakile, Joan M Chow, Kathleen J Gustafson, Heather Jost, Jeffrey M Schapiro, Susan Novak-Weekley, Anthony Tran, Jim H Nomura, Victor Chen, Manie Beheshti, Townson Tsai, Karen Hoover, Gail Bolan

Infectious Disease Advisor material
Journal of Clinical Infectious Diseases abstract


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