Determination of the cut-off value of S-iELISA
The cut-off value of S-iELISA was determinated as described previously with some modifications (Li et al., 2015). Forty SADS-CoV-negative serum samples were tested using the S-iELISA as described above to determine the cut-off value. Each sample was tested three times, and the mean OD450nm value of SADS-CoV-negative serum samples plus three standard deviations (SDs) was used to calculated the cut-off value. The OD450nm value of serum samples showing greater than or equal to this cut-off value were considered SADS-CoV-seropositive. This calculation gives 99% confidence that all negative values will fall within the defined range.