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.