Anyone who becomes infected with Covid-19 can transmit the disease even before symptoms appear. Only a test can then find out whether someone carries the virus. And it obviously plays a major role in how the material for this test is obtained. This is what researchers at the University of Maryland/USA found out. To do this, they compared tests of nasal swabs (so-called midturbinate swabs, two centimeters deep, abbreviation MTS) with saliva samples.
Regular saliva and swab samples in suspected cases
The team had already started testing healthy volunteers in May 2020. To do this, they took saliva samples weekly to detect a possible Covid-19. In the asymptomatic volunteers who tested positive, the researchers found that those patients typically showed symptoms a day or two later. “So we wondered if saliva is a better way to detect pre-symptomatic patients than traditional nasal swabs,” said co-author Donald K. Milton, professor of occupational and environmental health at the University of Maryland.
To do this, the team examined data from an accompanying study of close contacts of people with confirmed cases of Covid-19. 58 such contacts provided saliva and nasal swab samples every 2 or 3 days during their quarantine period. “All samples were generated using reverse transcription polymerase chain reaction [RT-PCR] tested in real time,” says Milton, “to detect Sars-CoV-2 and measure how much viral RNA was in the samples. We then analyzed how these results changed in the days before and following the onset of symptoms.”
Result: Saliva initially twelve times more accurate
There were similar virus detection rates for saliva and nasal swab, but sensitivity varied significantly with time from onset of symptoms. Within the first two days, saliva had a 12-fold higher probability of virus detection and 3.2-fold higher RNA copy numbers than MTS. The researchers see the results of their work as very important, especially as new variants with shorter incubation times, which may be detected in saliva rather than nasal swabs, are emerging, particularly “in large-scale screenings in schools and workplaces as a means of improving screening rates and for early detection,” says Milton.
In addition, there are other advantages, according to the study. Saliva sampling is easier than deep nasal penetration, which also requires the intervention of trained professionals, can cause patient discomfort, and poses a greater risk to medical staff during sample collection. This is because it can lead to coughing and sneezing, spreading viral particles as a result of swabbing the sensitive nasal passages.
Limitations: Small group, other studies with different results
However, there are some factors that limit the conclusions of the study, according to the researchers. Thus, the contacts enrolled in this study who eventually tested positive for SARS-CoV-2 developed mild and, in some cases, transient infections. Some of these mild infections would have gone undetected by less frequent testing protocols and might not have posed a risk for onward transmission. Only 14 contacts tested positive, a very small sample size. And earlier studies came to different conclusions. The team from the University of Maryland still sees the work as an important reference for saliva samples. For the study, however, this was only possible with complex PCR tests. Future research should therefore focus on the development of rapid saliva tests with high sensitivity and specificity.
