Phase 1: Laboratory testing
COVID-19-related VOCs
Data regarding the dogs’ response and the main characteristics of all the samples utilised during both the test and retest phases are reported as supplementary material (Supplementary Table The combined sensitivity and specificity for all three dogs during the test phase was Se = 93% (95% CI = 84–103%) and Sp = 99% (95% CI = 97–101%). The combined sensitivity and specificity for all three dogs during the retest phase was Se = 83% (95% CI = 69–97%) and Sp = 97% (95% CI = 94–99%). Overall and individual Se and Sp are reported in Table Canine ability to detect COVID-19-related VOCs in the laboratory test and retest.Measured and simulated overall positive predictive values (PPVs) and negative predictive values (NPVs) with 95% confidence intervals.Overall, discrepancies between the dogs’ response and the RT-PCR result were observed for 14/360 samples (3.9%), 50% of which were positive. Seventy-one per cent (n = 10) of the discrepancies were found in the retest (Table Main biological characteristics of samples and experimental phase (test/retest) associated to discrepancies between SARS-CoV-2 RT-PCR and the response from the three dogs.As for test–retest reliability overall, an 88% agreement was found between test and retest, which refers to the percentage of trials for which there was no discrepancy between the two phases. Regarding the Spearman’s correlation, one dog (Nala) yielded a Spearman’s r = 1 (100% correct responses). However, the percentage agreement for Nala was 91%. When considering the other two dogs individually, a moderate to strong test–retest reliability was found for Otto (90% agreement, Spearman’s r = 0.7, p = 0.035, 95% CI 0.042–0.917) while the reliability was lower for Helix (83% agreement, Spearman’s r = 0.5, p = 0.082, CI − 0.093 to 0.849).As for GzLM model results, none of the factors included was a significant predictor for the dogs’ correct response (all p > 0,05, data non shown).