A study carried out in the Northern Metropolitan Primary Care confirms that the antigen tests to detect covid-19 must be carried out with a nasal and not an oral sample

Only 31.7% of the positive ART performed with a nasal sample also gave a positive result with the oral sample. Samples from 300 CUAP users and from centralized points where covid-19 detection tests are carried out have been analyzed.

A study carried out at the USR Metropolitana Nord shows that rapid antigen tests (TAR) carried out from nasal samples give better performance than those carried out with oral samples (from saliva) to detect positive cases of covid -19. To reach this conclusion, samples from 300 users of primary care emergency centers (CUAP) and centralized points where detection tests for this disease are carried out have been analyzed. Practically all the participants (95%) had symptoms compatible with covid-19.

Specifically, both types of samples were collected for all the people participating in the study. On the one hand, a TAR was performed with the nasal sample and, on the other, a TAR was performed with the saliva sample from the mouth. In the event that the results of the two tests did not coincide, the person was asked if a new sample could be taken, to be analyzed by means of an rt-PCR test at the North Metropolitan Clinical Laboratory.

Looking in detail at the results obtained, it stands out that only 31.7% of the ART performed with a nasal sample gave a positive result were also positive with the oral sample. Likewise, in cases with discordant results, in which the ART performed with the nasal sample indicated that the result was positive for covid-19 but with the saliva sample it was negative, the PCR confirmed that they were positive. That is, whenever the ART performed with a nasal sample indicated a positive result, the PCR confirmed this positive, despite the fact that the ART performed with a saliva sample indicated that they were negative.

To ensure that the saliva samples were not contaminated, only volunteer participants who had not taken food, drink or gum, or smoked, in the 30 minutes prior to the collection of the oral sample were included in the study.

Taking into account some claims that the results of a rapid antigen test to detect covid-19 could vary depending on whether they were made from a nasal or oral (saliva) sample, Dr. Núria Prat, director of the North Metropolitan Primary Care of the ICS, explains that “it was decided to launch this study to confirm the validity of taking a saliva sample to carry out ART”. And he highlights the role of primary care in research of this type: "the fact of having direct access to a large volume of the population has allowed us to carry out the study in a very short time, as well as to obtain valid results very quickly".

For her part, Concepció Violán, one of the main researchers of the study, values ​​the rapid organizational capacity of the North Metropolitan Primary Care to develop this study. “There has been a great collaboration and motivation to carry out the study between the professionals of the CUAPs, those of the centralized points to carry out tests for the detection of Covid-19, of the Centralized Covid Support Unit and also the Clinical Laboratory of the North Metropolitan”, he points out. And he adds: "we appreciate the extra effort that has been made to carry out the study at this time of the height of the pandemic".

Thus, this study confirms that the sampling for rapid antigen tests is being done correctly in the nasal area. The study has had the participation of the Northern Metropolitan Primary Care of the ICS, the Northern Metropolitan Clinical Laboratory, the Germans Trias i Pujol University Hospital, the Jordi Gol i Gurina University Institute Foundation for Research in Primary Care (IDIAPJGol) , the Hermanos Trias i Pujol Research Institute (IGTP), the IrsiCaixa AIDS Research Institute and the Fight Against AIDS and Infectious Diseases Foundation. The publication has been made in the first instance in MedRxiv, where this study has been collected, with the article Comparison between mid-nasal swabs and buccal swabs for SARS-CoV-2 detection in mild COVID-19 sufferers.