2.2 Analysis
Our analysis was performed with R version 4.02 and used the same approach as our previously published analysis examining the performance of RIDT in a clinical setting.10 Binomial logistic regression models were used for adjusted associations. Adjusted models were fit for predicting improvements in sensitivity and specificity based on age, gender, number of days from symptom onset, presence and absence of individual signs and symptoms (chills, cough, sore throat, malaise, myalgia, runny nose, nasal congestion, headache), presence of an influenza-like illness (ILI), illness severity (mild, moderate, or severe), influenza vaccination status, co-detection of other pathogens, and season (early, peak, or late). Specimens meeting inclusion criteria that had missing information were included in the overall sensitivity and specificity estimates, but they were removed from the adjusted model. We defined ILI as having a fever with cough and/or sore throat [23]. A quadratic term for age was not included because it did not significantly improve the model (LRT p = 0.46). Interaction terms between age and days from onset were also considered but ultimately excluded due to lack of significance (LRT p = 0.11). Some students had multiple specimens present in the dataset. Specimens, even from the same student, were assumed to be independent for testing and modelling.
RESULTS
Of the 2,378 total student ARI episodes between January 5, 2015 and March 12, 2020, 2,368 (99.6%) were included in this analysis. Nine specimens (0.38%) were excluded from the overall analysis because the child’s symptoms began more than seven days before the samples were collected, and one (0.04%) was excluded because an RIDT was not performed. For the adjusted model, 17 records were excluded because they were missing severity of illness (n=12; 0.50%) or were marked as missing the number of days from symptom onset due of an invalid test date (n=5; 0.21%). Three (0.13%) specimens had inconclusive Sofia test results. These were imputed as false positives in specificity analysis as all three had negative PCR results.