An evaluation of the relative sensitivities of the Venereal Disease Research Laboratory Test and the Treponema pallidum particle agglutination test among patients diagnosed with primary syphilis

Background: Because definitive methods for diagnosing primary syphilis are limited, it is important to optimize the sensitivity of serodiagnosis. Objective: To determine the most sensitive testing approach to the diagnosis of primary syphilis, using the commonly available serologic tests: the Venereal Disease Research Laboratory (VDRL) test and the Treponema pallidum particle agglutination (TP-PA) test. Methods: Sensitivities of 2 serologic testing strategies for primary syphilis were compared among 106 darkfield-confirmed cases treated in San Francisco from January 2002 through December 2004. Results: The sensitivity of the diagnostic strategy using VDRL confirmed by TP-PA was 71% (95% CI, 61%-79%). Substituting Rapid Plasma Reagin test for VDRL in a subset of 51 patients produced the same sensitivity (71%; 95% CI, 56%-83%). The sensitivity of TP-PA as the first-line diagnostic test was 86% (95% CI, 78%92%). The sensitivity of the former approach was significantly lower among HIV-positive patients, compared with HIV-negative patients (55% vs. 77%, P = 0.05). Conclusions: The TP-PA test as the first-line diagnostic test yielded higher sensitivity for primary syphilis than did the use of the currently recommended strategy.