Determination of repeatability of S-iELISA
The reproducibility within and between runs of S-iELISA was evaluated, as described previously with some modifications (Li et al., 2015; Lin et al., 2018). Six control serum samples (5 IFA-positive samples and 1 IFA-negative samples) were selected for reproducibility experiments. For intra-assay reproducibility, each serum sample was tested in three replicates on the same plate within the same occasion. For inter-assay reproducibility, each serum sample was tested in three replicates on different plates of different occasions. The results were presented as the coefficient of variation (CV), which is the ratio of the standard deviation (SD) to the mean OD450nm value of each group of samples (S). A CV value criterion of 10% was used to meet the repeatability requirement of the test.