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

Background: Strongyloidiasis is a chronic parasitic disease that results in relevant human morbidity, caused by the nematode Strongyloides stercoralis. This nematode has a unique and complex life-cycle. There is so far no perfect test for this helminthiasis. Rapid immunochromatographic tests (RDTs) are of interest, specifically due to their feasibility for use in the field, where public health control of strongyloidiasis is recommended. The aim of this study was to evaluate two novel RDTs, one detecting immunoglobulin (Ig) G and the other detecting IgG4, based on a combination of recombinant antigens. The primary objective was to estimate the sensitivity and specificity of these RDTs, and the secondary objective was to assess ease of interpretation.

Methods: Serum samples stored in our biobank with available matched results for at least one fecal (i.e. agar plate culture or PCR) and one serology test (i.e. enzyme-linked immunosorbent assay [ELISA] or indirect immunofluorescent antibody test [IFAT]) for S. stercoralis, were selected for this study. Those with at least one positive result for the fecal test were considered to be true positives (irrespective of the serology), while true negatives were those with negative results for both the fecal and serology tests. The results of the RDTs were read independently by two laboratory technicians. When disagreement over the results occurred, a third reader was involved, and the final result for each test was based on consistent results from two readers. Estimates were reported along with the 95% confidence intervals (CI). Regarding the secondary objective, agreement between two independent readers was calculated with Cohen's kappa statistic (κ).

Results: A total of 90 serum samples were tested. Sensitivity of the IgG- and the IgG4-RDTs was 91.1% (95% CI 78.8-97.5) and 77.3% (95% CI 62.2-88.5), respectively. Specificity was 91.1% (95% CI 78.8-97.5) for the IgG-RDT and 100% (95% CI 92.1-100) for the IgG4-RDT. Agreement between readers was excellent (Cohen's κ = 0.96, 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. The results of both RDTs were easy to interpret 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 purposes/contexts.