Around the antigen tests: nasal or throat sample?

Around the antigen tests: nasal or throat sample?

Now that we are approaching a new management of the pandemic in which COVID-19 tests in the health system will be reserved for vulnerable peopleAs with preventive isolation, it is important to know what information the sales tests in pharmacies can provide us with, and which are the most reliable.

A few months ago, when the demand for antigen self-tests skyrocketed due to the drastic increase in coronavirus cases in the sixth wave and the winter celebrations, multiple questions arose. Among various questions, a debate arose strongly about whether it was better to collect the sample by applying the swab to the nose, as indicated in the instructions of almost all of the antigen tests marketed in Spain (except those that work with saliva samples ), or, on the contrary, it was more reliable to obtain the sample from the throat.

Several factors contributed to generating this diagnostic controversy. On the one hand, the sixth wave meant the expansion of the omicron variant over the others in our country. Preliminary studies, with quite a few limitations, had observed that the viruses of this variant proliferated much faster in the tissues of the bronchi and throat than in the lungs or the nose. In this way, the detection of the coronavirus through nasal samples could be less reliable, as it appears later there than in the throat.

On the other hand, several scientists and sanitary They commented on social networks about their different experiences when using self-test for antigens with samples taken from the throat or nose. When they tested after swabbing the throat, rather than the nose, or when they combined the throat swab with the nose, more positive results appeared than if the swab was exclusively nasal.

Furthermore, at that time appeared various preprints –scientific articles not yet peer-reviewed or published in scientific journals– that suggested that self-diagnostic tests using antigens offered greater sensitivity (ability to detect positive cases for SARS-CoV-2 of the omicron variant) if the person took a sample throat or throat combined with nasal, instead of the conventional nasal sample.

At that time, health institutions such as the United States Food and Drug Administration (FDA) and the Spanish Medicines Agency (AEMPS) spoke in support of taking the samples as indicated by the manufacturer. In the case of the AEMPS, on the web "General information on COVID-19 diagnostic tests" clarifies that: "The type of sample required is indicated in the instructions for use of each product. A sample other than the one specified in these instructions should never be used because it increases the risk of obtaining an erroneous result."

The FDA, for its part, reported through different channels (including social media) that: "Currently available home antigen tests are only licensed for use with nasal swabs. We do not have any data yet to suggest that throat swabs are an accurate or appropriate method for test at home.

As in many other areas of the pandemic, scientific uncertainty reigned once again due to the lack of reliable and rigorous data on the performance of antigen self-tests with infections caused by omicron. The various antigen tests marketed for nasal swabs have been designed and evaluated exclusively from swabs collected from the nose of people infected with other than omicron variants, so there is no guarantee that they will work for other types of swabs.

An important factor to consider about these tests is the pH of the sample. That is, its acidity or alkalinity. The results of these tests are reliable within certain pH ranges, but outside of them the chances of giving erroneous results increase.

The throat is usually a more acidic environment (lower pH) than the nose and, in addition, it is more susceptible to changes depending on the food and drink that the person previously ingested. Citrus or coffee consumption can further lower the pH of the throat. Could it be that the positive throat swab antigen test results shared on social media were partly false positives? It is a possibility that cannot be ruled out.

All these factors made it impossible to know with certainty whether the reliability of the nasal antigen self-test was superior when the samples were taken through the throat. To change the indication of these tests and to pass the authorization of the drug agencies, it was necessary to carry out studies that rigorously evaluate this different method to obtain the sample, a process that requires time and money.

Is there now new evidence to support throat swabs for nasal antigen testing? At the moment, no. In fact, preprints have appeared that question whether taking throat samples exclusively is a good idea.

On January 19, a preliminary study in which the results of PCR tests and rapid antigen tests (of the BinaxNOW brand) were compared according to the origin of the sample, which could come from the nose, throat or inside the cheek. Tests were carried out on 731 people, of which 40.5% tested positive in the PCR tests. 98.5% of the positive samples analyzed contained the omicron variant.

The researchers found that samples from the inside of the cheeks were by far the least reliable, with almost all positive cases by other methods (91%) appearing negative. On the other hand, throat samples had lower sensitivities (74%) compared to nasal samples (97.7%) for samples with high viral loads detected by PCR.

However, the combination of nasal and throat sampling offered slightly better reliability. Given these results, the authors explained in their conclusions that "throat swabbing performed poorly compared to standard nasal swabbing. Even with the prominent clinical feature of pharyngitis in people with omicron, our data do not support substituting nasal swabs with throat swabs for diagnosis".

Other preprint, published on January 21, compared the results of different antigen tests with samples obtained from the nose, throat or both. Although the results of the nasal or throat samples varied according to the brand of the test, all of them offered greater sensitivity when the throat sample was combined with another nasal.

Although more studies are needed, the evidence suggests that combining both types of samples (nasal and throat) provide greater guarantees in antigen tests. This is something that has been recommended in the UK from mid 2020 for some specifically indicated tests. It remains to be clarified whether this increased reliability per pooled sample is a phenomenon that occurs for all antigen tests, or depends on each one. To do this, it will be necessary for manufacturers to carry out specific tests on their products and for drug agencies to give their approval.

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