Based on ROC analysis of the SIgA-ELISA, the S/P ratios of nasal swab samples varied from a minimum of0.056 to a maximum of 0.205 for negative sample and from a minimum of 0.069 to a maximum of 1.334 for positive swab. A cut-off of 0.107 was found to give an optimal
result with a DSP of 94.4% and a DSN of 97.0% (Fig. 3).