Nasopharyngeal or throat swab are uncomfortable invasive procedures, which increase the infection risk to health care workers. Besides being time-consuming, it is labor intensive and depends on the availability of supplies such as nasal swabs and transport media. Therefore, an alternative diagnostic specimen which is noninvasive, easy to collect, cost-effective and having low COVID-19 infection risk for healthcare workers, is urgent necessity. For this purpose, saliva [5-8] and mouthwash samples [3,4,9] were used in different studies [3-9]. Although there are numerous studies with saliva, only limited studies were found with mouthwash samples [3,4].
In our study we investigated the performance of MyMagiCon-RW100®, a new method which enables concentrating GMW samples for increasing the sensitivity of detecting microorganism by RT-PCR. We also compared the diagnostic utility of GMW and concentrated GMW samples with gold standard sample, NPS.
Overall, concentrated GMW-based SARS-CoV-2 testing showed moderate concordance and sensitivity with nasal swab samples. However, the PCR of concentrated GMW samples yielded better results than the PCR of NPS in two centers. In all centers concentration of GMW samples increased the sensitivity of COVID-19 detection. The PCR positivity between GMW and concentrated GMW increased in 16.6 % (ranging from 8 to 38.5% in different centers).
When the results of each center examined separately, the percentage of PCR positivity of NPS, GMW and concentrated GMW showed variability. There can be several reasons for this variability. First of all, in some centers only patients with positive NPS were included. However, there can be COVID-19 patients with negative PCR results in NPS samples which was observed in centers which included all NPS samples either positive or negative by PCR. Secondly, we have shown that different brands of drinking water used across the centers, which affected the RT-PCR amplification, is another explanation for false negative results. Therefore, adding a standard drinking water, which is shown not to inhibit PCR, as a part of MyMagiCon-RW100® kit can eliminate false negative PCR results. It will be beneficial, standardization of the collection of GMW sampling technique and the mouthwash fluid, in future studies.
Biber et al  also investigated the utility of using mouthwash (MW) samples for the detection of SARS-CoV2. The patients either asymptomatic or with mild symptoms were included in the study. Although 84.6% (116/137) of oronasopharyngeal (ONP) swabs were positive by at least one of the genes (N, E, R), MW detected 70.8% (97/137) positivity. Unfortunately, MW samples missed samples especially NPS samples with Cq value >30. The median of Cq values of all positive ONP swabs (all three genes) was found as 30 compared to a median of 32 of positive MW. They also compared the results of different fluids (saline, distilled water, commercial mouthwash containing alcohol and commercial mouthwash without alcohol) for mouth rinsing. The result of water was similar to saline in contrast to other solutions, which had lower detection rate.
There are several studies which compared saliva to NPS and in most of them saliva sensitivity was found either comparable to that of NPS [10-16] or higher than NPS [17-19]. Although there are several studies which compared saliva with NPS, only symptomatic patients were included in most of them. There are limited study investigating the performance of saliva compared to NPS in asymptomatic individuals [19-22]. Alkhateeb et al  compared saliva and NPS of 33 symptomatic and 12 asymptomatic known SARS-CoV-2-positive patients. Though, saliva showed lower sensitivity (36%) in asymptomatic patients than symptomatic patients (80%), it detected infections with lower Ct values. Opposite to the results of A1, there are researches in which saliva has same or sometimes higher sensitivity for detecting asymptomatic carriers compared to NPS [21-23]. Savela et al  evaluated the sensitivity and sample type for detecting early infections of COVID-19 in asymptomatic individuals. They quantified SARS-CoV-2 RNA viral loads in anterior-nares nasal swabs and saliva samples which were obtained twice-daily and found that SARS-CoV-2 RNA first appears in saliva compared to nasal-swab samples. In other studies it has been demonstrated that SARS-CoV-2 RNA can appear in saliva 24–48 h prior to detection by nasopharyngeal swabs [19,24,25], and 1.5–4.5 days prior to detection by anterior nasal swabs . In the study of Yokoda et al  high sensitivity and specificity was detected in NPS and saliva specimens of 1924 asymptomatic individuals.
One of the limitation of our study is including only symptomatic patients into the study. Overall it is estimated that 17 to 30% of SARS-CoV-2 positive individuals remain asymptomatic [25,26] . As SARS-CoV-2 can spread from individuals before symptom onset and from asymptomatic individuals, the performance of GMW sampling in asymptomatic population must be also investigated.
In conclusion, the GWM sampling is a simple, rapid, cheap and self-collection method. According to the results of this study, concentrated GMW can be an alternative sampling method to NPS in rapid and accurate diagnosis of COVID-19. Standardization of the method by using water with low ion concentration will increase the performance of the test and lower the variability of the results. Additional studies are needed including both symptomatic and asymptomatic patients for accurate evaluation of GMW and concentrated GMW sampling as a screening test for COVID-19.