University of São Paulo: Saliva test allows to detect differences in coronavirus viral load

ANDThe saliva exam developed at USP is able to point out a difference in viral load not detected in tests carried out with samples taken from the nose and pharynx. Tests done on patients with covid-19 suggest, for example, that men have a higher viral load in their saliva than women, something that does not show up in nasopharyngeal swabs. According to the researchers, this difference needs to be confirmed by new studies, involving a greater number of cases and in other contexts.

The result was obtained in a preliminary study coordinated by the Center for Research on Human Genome and Stem Cells (CEGH-CEL), of the Institute of Biosciences (IB) at USP. Researchers have developed a rapid test to detect the coronavirus, which can be done in places with little infrastructure, with less cost. Data from the work are presented in preprint (previous version of a scientific article, not yet revised) published on the medRxiv website on June 9th.

“Our initial objective was to develop a diagnostic test based on the RT-LAMP technique, which would be cheaper, that would not depend on imported inputs or availability of sophisticated laboratory infrastructure, and that would allow a quick diagnosis, within a 24-hour period”, explains the researcher Gerson Kobayashi, from the CEGH-CEL, the first author of the work. RT-LAMP is a technique for amplification and detection of genetic material similar to RT-PCR, but it is faster, cheaper and does not require specialized equipment for its execution.

“These characteristics would be important to reduce the risk of transmission, especially given the limited access to international supplies and the need for rapid testing imposed by the pandemic. It is noteworthy that the test uses saliva, which is easily accessible and does not require specialized professionals for collection, simplifying the entire process.”

To develop the methodology, the researchers used about 150 saliva samples collected from patients with symptoms of covid-19, in collaboration with the Institute of Tropical Medicine (IMT) of the Faculty of Medicine of USP (FMUSP) and the company Prevent Senior . “To determine its diagnostic potential, we also analyzed the viral load in these salivas and compared it with data from the RT-PCR test performed on nasopharyngeal swabs, which is the current gold standard for the diagnosis of covid-19”, says the researcher.

According to Kobayashi, in the saliva test, the interpretation of the results can be made in the change of color of the tubes that occurs during the reaction. “We have developed a solution that stabilizes the virus genome in saliva, ensuring a high rate of detection by RT-LAMP and eliminating time- and resource-consuming isolation and purification procedures for this material, which are necessary for standard RT-PCR testing. ”, he reports. “This solution is made up of accessible ingredients available in most laboratories.”

Viral charge
The research found that the amount of virus in saliva is greater in the first days of symptoms and drops considerably within ten days, which corresponds to recent data on the transmissibility of the disease. “We also found that men have about ten times more virus in their saliva than women, particularly in individuals under 40 years of age, which may be related to the greater chance of developing severe symptoms seen in men,” explains Kobayashi. “We didn’t get these results for nasopharyngeal swabs, which suggests that saliva is better correlated with the dynamics of virus transmission and infection.”

According to Professor Maria Rita Passos-Bueno, from the IB, who coordinated the study, preliminary data show that men, especially younger ones, are more likely to spread the virus, contributing more to its transmission than women.

“Since women have milder clinical pictures of covid-19 than men, a hypothesis for this difference in viral load is that the women’s immune system is more efficient in eliminating the virus, but this also needs to be proven by epidemiological studies and laboratory.”

“However, this result needs to be confirmed by new studies, involving a greater number of cases and in other contexts”, he points out. Only 49 samples were analyzed in this comparison between the sexes.

Maria Rita Passos-Bueno – Photo: Personal archive

In addition to the possibility of quick testing, less costly, and capable of being performed in places with little infrastructure, the researchers emphasize that testing in saliva may be more advantageous than tests based on nasopharyngeal swabs, since saliva seems to reflect better the biological and transmission characteristics of the sars-cov-2 virus. “For example, it is possible that a positive result in the nasopharynx does not necessarily indicate that the individual is in the transmission phase of the virus, which may depend more on the amount of virus in saliva and on the time since the onset of symptoms”, highlights Kobayashi. “Further studies must be done to confirm these hypotheses.”

Maria Rita and Kobayashi point out that due to its simplicity, speed and accessibility, the saliva test can be carried out periodically to better control the pandemic. “The number of data provided by saliva tests has been growing, and the more one understands the presence of the virus, the greater is the safety in using the results”, points out the IB professor. “Since the mouth is one of the main means of transmission of the coronavirus, an indication of a low viral load in saliva is important in controlling the pandemic, as a negative test may indicate less chance of transmitting the virus.”