Retrospective Review of Treponema pallidum PCR and Serology Results: Are Both Tests Necessary?

There has been a resurgence of syphilis diagnoses in Australia. We investigated whether our PCR test provides any additional diagnostic information over syphilis serology (chemiluminescence immunoassay [CMIA], particle agglutination [TPPA] assay, and the rapid plasma reagin [RPR] flocculation test). A retrospective audit of all PCR requests that came through our laboratory from January 2010 to June 2017 was conducted; data collected included age, gender, site of swab, and results from PCR, syphilis serology, and herpes simplex virus 1 (HSV-1) and HSV-2 PCRs. A total of 441 PCR tests were performed; on average, 3 PCRs per month were requested in 2011, and this rate increased to 17.2 requests per month in 2017. A total of 323 patients had both PCR and syphilis serology performed, with 67% of swabs taken from the genitals. PCR gave positive results for 61/323 (19%) patients; of these 61 patients, 59 (97%) also had positive syphilis serology results ( PCR sensitivity, 68%; specificity, 99%; positive predictive value, 97%; negative predictive value, 89%). Syphilis serology was positive for 91/323 patients (28%); of these 91 patients, 61 (66%) were also PCR positive (syphilis serology sensitivity, 97%; specificity, 88%; positive predictive value, 60%; negative predictive value, 99%). The Cohen's kappa value was 0.74, indicating substantial agreement between the two tests. Our results show that most patients with positive PCR results also had positive syphilis serology. Therefore, PCR adds little clinical value over serology for the diagnosis of syphilis in certain clinical settings.