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Background: WHO recommends antenatal syphilis serologic testing for all pregnant women, but only 10% of Malawians receive recommended screening. Dual rapid diagnostic tests (RDTs) incorporating syphilis and HIV testing on one cartridge hold potential to detect and treat syphilis in pregnant women who receive HIV testing. We evaluated the field performance of the syphilis component of a dual HIV/syphilis RDT (Chembio DPP HIV-Syphilis Assay) in Malawi.
Methods: During July 2014—November 2015, 1798 pregnant women attending their first antenatal visit at Machinga District Hospital were recruited. In addition to the Malawi-approved single RDTs for HIV and syphilis (Alere Determine), participating women received the dual HIV/syphilis RDT under evaluation and had blood drawn for Rapid Plasma Reagin (RPR) and Treponema pallidum particle agglutination (TPPA) reference testing at CDC. We calculated field performance characteristics of both treponemal-only syphilis RDTs—the approved single test and novel dual test—relative to TPPA and TPPA/RPR results.
Results: Of 1798 women screened, 1784 (99.2%) had complete rapid- and reference-testing results. Of these, 80 (4.5%) were TPPA+ and 46 (2.6%) were TPPA+/RPR+. Using only TPPA as the standard, the dual RDT under evaluation had 68.8% sensitivity and 99.6% specificity, whereas the single RDT had 63.8% sensitivity and 99.8% specificity. Using active syphilis infections (TPPA+/RPR+) as the standard, the dual RDT had 91.3% sensitivity, and the single RDT had 84.8% sensitivity. When analysis was limited to those most likely to vertically transmit syphilis (TPPA+/RPR+, titer ≥ 1:8), the sensitivity of the dual RDT improved to 100.0% (31 of 31 detected) and the single RDT was 87.1% (27 of 31 detected).
Conclusions: The syphilis component of the dual RDT had a lower field sensitivity than anticipated but performed comparably to the single RDT for syphilis currently in use. The dual RDT performed very well among women likely to vertically transmit syphilis and suffer adverse pregnancy outcomes.