The use of Western blotting as the confirmatory test for syphilis in patients with rheumatic disease

Objective. As a direct or indirect result of antiphospholipid antibody prod uction, subjective laboratory interpretation, and false positive results, t he common serologic tests for syphilis have been inherently inaccurate diag nostic tests in patients with systemic lupus erythematosus (SLE) and other autoimmune diseases. We assessed the diagnostic accuracy of syphilis testin g in patients with SLE and other autoimmune diseases using the treponemal W estern blot (TWB) as the gold standard. Methods. A prospective cohort study carried out at a tertiary care medical center. We studied 107 patients with autoimmune disease, 50 with at least o ne positive serologic test for syphilis and 57 disease matched controls. Pr ior to enrollment all eligible patients underwent a clinical assessment per formed by at least 2 rheumatologists to confirm a diagnosis of rheumatic di sease. All subjects underwent serologic testing, in blinded fashion, for sy philis using the rapid plasma reagin test (RPR), Venereal Disease Research Laboratory test (VDRL), fluorescent treponemal antibody absorption test (FT A-ABS), and the TWB. Results, Eighty-seven percent of the patients studied were female, the mean age was 46.5 years, and the most common diagnosis at the time of enrollmen t was SLE. Using the TWB as the gold standard diagnostic test for syphilis, the sensitivity, specificity, and positive predictive values for each syph ilis test were calculated, The sensitivity and specificity For the RPR in p atients with rheumatic disease was 62.5% (95% confidence interval 24.5 to 9 1.5%) and 91.9% (95% CI 84.2 to 96.2%), respectively. The sensitivity and s pecificity for the VDRL were 37.5% (95% CI 8.5 to 75.5%) and 89.9% (95% CI 81.8 to 94.8%), respectively. Confirmatory syphilis testing using the FTA-A BS showed a sensitivity of 100% (95% CI 68.6 to 100%) and a specificity of 67.7% (95% CI 57.4 to 76.5%). Eight patients tested positive for syphilis b y Western blotting. For the FTA-ABS test, there was a significantly higher number of false positive results (n = 32) compared to false negative result s (n = 0), p < 0.0005. Conclusion. The FTA-ABS is not an accurate confirmatory test for syphilis i n patients with SLE and other autoimmune diseases. While a negative FTA-ABS may exclude syphilis infection in the majority of cases, a positive FTA-AB S test result cannot assuredly confirm syphilis infection in this populatio n. Western blotting is an accurate confirmatory test for syphilis and may b e necessary to unequivocally discern the immunological response of syphilis from that of an underlying auto-immune disease.