Evaluation of novel recombinant antigen-based (NIE/SsIR) immunochromatographic rapid tests for Strongyloides stercoralis: an accuracy study

Abstract

Background. Strongyloidiasis by Strongyloides stercoralis is a parasitic infection with a complex cycle, causing relevant human morbidity. There is no perfect test for this helminthiasis. Rapid immunochromatographic tests (RDTs) are of interest specifically for their feasibility of use in the field, where public health control of strongyloidiasis is recommended. Aim of this study was to evaluate two novel RDTs, one detecting IgG and the other IgG4, based on a combination of recombinant antigens. Primary objective was to estimate sensitivity and specificity of these RDTs. Secondary objective was ease of interpretation. Methods. We selected serum samples stored in our biobank, with available matched results of at least one fecal (i.e. agar plate culture or PCR) and one serology test (i.e. ELISA or IFAT) for S. stercoralis. Cases with at least one positive fecal test were considered true positive (irrespective of the serology), while true negatives were cases with negative fecal and serology tests. The results of the RDTs were read independently by two laboratory technicians. In case of discrepant result, a third reader was involved. Final result for each test was based on consistent results from two readers. Estimates were reported along with the 95% confidence intervals. For the secondary objective, agreement between two independent readers was calculated with Cohen’s k. Results. 89 sera were tested. Sensitivity of the IgG and of the IgG4 RDTs was 89.1% (95% CI 76.4-96.4) and 75.6% (95% CI 60.5-87.1), respectively. Specificity was 90.9% (95% CI 78.3-97.5) for the IgG, and 100% (95% CI 92-100) for the IgG4 test. Agreement between readers was excellent (0.96% k, 95% CI 0.86-1.08). Conclusions. The IgG-RDT demonstrated higher sensitivity, and could hence be preferred for individual diagnosis, whereas the excellent specificity of the IgG4-RDT could be preferred for prevalence surveys in endemic areas. Both RDTs demonstrated easy to be interpreted, based on excellent agreement between readers. Large prospective studies should follow, to confirm these findings and to validate the use of either RDT for specific purpose/context.