The role of mouthwash sampling in SARS-CoV-2 diagnosis

Abstract Background The current practice of COVID-19 diagnosis worldwide is the use of oro-nasopharyngeal (ONP) swabs. Our study aim was to explore mouthwash (MW) as an alternative diagnostic method, in light of the disadvantages of ONP swabs. Methods Covid-19 outpatients molecular-confirmed by ONP-swab were repeatedly examined with ONP-swab and MW with normal-saline (0.9%). Other types of fluids were compared to normal-saline. The Cq values obtained with each method were compared. Results Among 137 pairs of ONP-swabs and MW samples, 84.6% (116/137) of ONP-swabs were positive by at least one of the genes (N, E, R). However MW detected 70.8% (97/137) of samples as positive, which means 83.6% (97/116) out of positive ONP-swabs, missing mainly Cq value>30. In both methods, the N gene was the most sensitive one. Therefore MW samples targeting N-gene, which was positive in 95/137 (69.3%), is comparable to ONP-swabs targeting E and R genes which gave equal results – 95/137 (69.3%) and 90/137 (65.7%) respectively. Comparing saline MW to distilled-water gave equal results, while commercial mouth-rinsing solutions were less sensitive. Conclusions MW with normal-saline, especially when tested by N gene, can effectively detect COVID-19 patients. Furthermore, this method was not inferior when compared to R and E genes of ONP-swabs, which are common targets in many laboratories around the world.