Scent dogs in detection of COVID-19: triple-blinded randomised trial and operational real-life screening in airport setting

Abstract

Objective: To estimate scent dogs' diagnostic accuracy in identification of people infected with SARS-CoV-2 in comparison with reverse transcriptase polymerase chain reaction (RT-PCR). We conducted a randomised triple-blinded validation trial, and a real-life study at the Helsinki-Vantaa International Airport, Finland.

Methods: Four dogs were trained to detect COVID-19 using skin swabs from individuals tested for SARS-CoV-2 by RT-PCR. Our controlled triple-blinded validation study comprised four identical sets of 420 parallel samples (from 114 individuals tested positive and 306 negative by RT-PCR), randomly presented to each dog over seven trial sessions. In a real-life setting the dogs screened skin swabs from 303 incoming passengers all concomitantly examined by nasal swab SARS-CoV-2 RT-PCR. Our main outcomes were variables of diagnostic accuracy (sensitivity, specificity, positive predictive value, negative predictive value) for scent dog identification in comparison with RT-PCR.

Results: Our validation experiments had an overall accuracy of 92% (95% CI 90% to 93%), a sensitivity of 92% (95% CI 89% to 94%) and a specificity of 91% (95% CI 89% to 93%) compared with RT-PCR. For our dogs, trained using the wild-type virus, performance was less accurate for the alpha variant (89% for confirmed wild-type vs 36% for alpha variant, OR 14.0, 95% CI 4.5 to 43.4). In the real-life setting, scent detection and RT-PCR matched 98.7% of the negative swabs. Scant airport prevalence (0.47%) did not allow sensitivity testing; our only SARS-CoV-2 positive swab was not identified (alpha variant). However, ad hoc analysis including predefined positive spike samples showed a total accuracy of 98% (95% CI 97% to 99%).

Conclusions: This large randomised controlled triple-blinded validation study with a precalculated sample size conducted at an international airport showed that trained scent dogs screen airport passenger samples with high accuracy. One of our findings highlights the importance of continuous retraining as new variants emerge. Using scent dogs may present a valuable approach for high-throughput, rapid screening of large numbers of people.

Keywords: COVID-19; Clinical trial; Infections, diseases, disorders, injuries; Public Health.

Figure 1

The purpose-built cubicle at the Helsinki-Vantaa International Airport. (A) The cubicle from the outside with the doors into the three sampling rooms. (B) Sampling room with a hatch for handing in the sample for the scent detection dog test. (C) A room for scent detection dog testing, showing two of the three hatches to the right. (D) White Shepherd, E.T., inside the test room, indicating the sample in the middle (No 2) as positive. During the validation, only three of the five scent track holes had cans with samples.

Figure 2

Triple-blinded study. Assistant A gives the track through a hatch in the wall to assistant B, who places it on the floor and, after the dog and dog handler C have completed their work, gives it to assistant G. The dog handler C announces the result to data recorder D, who instructs whether to reward the dog. The external evaluator E and assistant F follow the setup from a video screen (four cameras inside the cubicle) and verify the triple-blinded study conduct. blinded: the dog, handler C, assistants B, E, G. circles: red, SARS-CoV-2 reverse transcriptase-polymerase chain reaction positive; green, negative sample.

Figure 3

Flow chart of the study conduct.