Background Traditionally, most diagnoses of syphilis relies on serology and required testing for patient antibodies against cardiolipin (non-treponemal tests) as well as against treponemal antigens (treponemal tests). These serological tests must be performed in clinical laboratories, require considerable scientific resources and trained technicians to perform them, and test results often are not reported for several days. As a result, control in resource-poor countries has been problematic. Rapid tests provide a platform whereby patient antibodies can be quickly analysed (5–30 min) by their binding to immobilised antigens on a nitrocellulose membrane. Another advantage is that patient sera, blood, or plasma can be tested, thus requiring fewer resources and technician time to perform. One of the first rapid tests developed was the Abbot Determine (treponemal test results only) which has served as a useful rapid screening tool in some settings. Other treponemal only rapid tests include the Standard Diagnostics BioLine and the Fujirebio Espline. These rapid tests have a sensivitivity ranging from 92% to 100% and specificity ranging from 93.4 to 98.9% when compared to the TP-PA. The Eurostrip (Euromedi Equipment LTD, W. Harrow, UK), another treponemal only rapid test, was recently evaluated at CDC for a potential field study in Kenya. With 94 archived serum samples, the Eurostrip had a sensitivity of 98.6% and a specificity of 100% when compared to the TP-PA.
Methods and results A recent breakthrough in technology allows for modified cardiolipin to be attached to membranes, and the first generation of dual rapid tests were developed in collaboration with CDC: the ChemBio DPP Screen and Confirm (ChemBio Diagnostic Systems, Medford, New York, USA) and the Span Spirolipin (Span Diagnostics Inc., Surat, India). In a study with 1601 archived serum samples, the non-treponemal component of the ChemBio DPP had a sensitivity of 97.3% and a specificity of 98.6% when compared to the RPR; the treponemal component had a sensitivity of 97% and a specificity of 95.5% when compared to the TP-PA. In a similar study with 376 archived serum samples, the non-treponemal component of the Span Spirolipin had a sensitivity of 96.5% and a specificity of 97.7% when compared to the RPR and the treponemal component had a sensitivity of 97.3% and a specificity of 99.1% when compared to the TP-PA. These evaluations demonstrate that these dual rapid tests are as sensitive and as specific as traditional RPR and TP-PA tests.
Conclusions The emergence of dual rapid tests offers resource-poor countries the opportunity for improved point-of-care diagnostic capacity. Span has recently announced that they can market the Spirolipin dual test for <1 USD per cassette, which would make these tests affordable in many settings. Future developments in rapid testing include the addition of antigen(s) to also test for HIV infection, offering even more diagnostic options to control these diseases, especially among pregnant women.
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