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Evaluation of nonspecific treponemal test rapid plasma reagin in comparison with specific treponemal test immunochromatographic assay for screening healthy blood donors

Introduction: Serological tests for syphilis (STS) contributed greatly to the detection of Treponema pallidum infection in blood donors and especially in those who were not identified during the medical selection. Most of the blood centers use nontreponemal tests due to lesser cost, ease of performance, and their ability to pick up early primary stage infection, despite reported high false positivity. However, increasingly large number of blood centers have begun testing with the treponemal tests such as immuno-chromatographic assay (ICA) which is possibly easier and more objective than rapid plasma reagin (RPR), albeit slightly expensive. It is with this background that we undertook a head-to-head comparison of nontreponemal test RPR with treponemal test ICA in over 10,000 consecutive blood donor samples with confirmation of all reactive and discordant samples by fluorescent treponemal antibody absorption assay (FTA-ABS) as the gold standard. Materials and Methods: The study was conducted in the department of transfusion medicine in a large tertiary care hospital in India. Consecutive blood donors from July 2014 to January 2015 were evaluated simultaneously for antitreponemal antibodies by solid phase ICA (SD BIOLINE Syphilis 3.0, Alere Medical Pvt. Ltd., USA), RPR (Immutrep RPR, Omega Diagnostics, Scotland, UK) and FTA-ABS (Biocientifica SA, FTA-ABS, Argentina). Performances of both the assays were evaluated in statistical terms. Results: A total of 10,200 donor samples were evaluated. There were 10,124 confirmed concordant negatives. Thirty-eight samples were concordant positive. Thirty-eight samples (thirty samples were ICA positive and RPR negative and eight samples were RPR positive and ICA negative) were discordant. The sensitivity of ICA was higher (93%) as compared to that of RPR (66%), whereas the specificity of RPR was higher (83%) than that of ICA (66%). Positive predictive value (PPV) of both the tests was similar while negative predictive value (NPV) of ICA was higher (86%) than that of RPR (60%). Youden's index was 0.81 for ICA whereas it was 0.62 for RPR. Conclusion: ICA has a better sensitivity than the RPR and thus seems to be more appropriate test for screening in blood donors.