Receiver-Operating Characteristic Analysis

We performed receiver-operating characteristic (ROC) analysis with the aim of defining an optimal Ct cutoff to distinguish symptomatic norovirus infections and asymptomatic infections. We defined a reference positive group and reference negative group based on the presence of AGE symptoms and timing of specimen collection. Reference positive included specimens collected from symptomatic cases within 3 days of symptom onset. Reference negative included specimens collected from asymptomatic controls regardless of the timing of specimen collection. All specimens, including reference negatives, were positive for norovirus by real-time RT-PCR and confirmed by sequencing. We conducted ROC analyses separately for norovirus GI and GII. The Youden index (sensitivity+specificity-1) was used to identify an optimal cutoff for the Ct value [22–24].