Nasopharyngeal swabs more effective than saliva samples for COVID-19 diagnostic testing according to current evidence

There is no need to use nasopharyngeal swabs for COVID-19 testing, mouth swabs can be used instead
Misleading: The nasopharyngeal swab procedure, in which a swab is inserted far into the nose to collect potentially infected cells from the back of the throat, is currently the most effective method for COVID-19 diagnostic testing. The alternative testing of saliva is still under evaluation, but preliminary data suggest that saliva contains less virus and the testing is less accurate than for nasopharyngeal specimens.
Due to its reliability and relative ease of specimen collection, the nasopharyngeal swab is considered the gold-standard procedure for collecting cells and secretions from the back of the throat for COVID-19 diagnostic testing. In contrast, the accuracy of less invasive testing of alternative specimens, such as saliva, is still under evaluation. Several laboratories have developed at-home kits for the collection of nasal mucus and saliva samples that are then shipped to a laboratory for analysis. However, specimen collection is a key step in clinical diagnosis, and experts have raised concerns about how potential flaws in at-home sample collection may impact the accuracy of these tests.

FULL CLAIM: “Modern Science can take a swab from the inside of your cheek and do a complete DNA mapping… Why not swab the inside of your highly contagious mouth?”


This 8 July 2020 Facebook post suggests that at-home testing for COVID-19 using mouth swabs, such as those used to collect DNA samples, could be just as effective as the nasopharyngeal swab method. Earlier versions of the claim questioned the need for nasopharyngeal sample collection in which “they have to stick a Q-tip through your nose to the back of your head to swab a sample of COVID” when it would be much easier to collect saliva samples because “a droplet of saliva will infect a whole village at one time”. Similar posts have been circulating widely on Facebook since the beginning of July 2020 and have received more than 90,000 interactions.

The post reflects a misunderstanding of how COVID-19 transmission and testing work. Interim Guidelines from the U.S. Centers for Disease Control and Prevention (CDC) for COVID-19 testing states that “proper collection of specimens is the most important step in the laboratory diagnosis of infectious diseases”. The distribution and quantity (viral load) of the SARS-CoV-2 virus across the respiratory tract depends on the patient and the type of specimen1. Because of the accuracy of the tests using nasopharyngeal specimens, this is considered the gold-standard collection method for COVID-19 diagnostic testing. The procedure involves inserting a long swab into each of the nostrils and through the nasal cavity until it makes contact with the upper part of the nasopharynx. The swab is then rotated to collect potentially infected cells and secretions for testing by reverse transcription polymerase chain reaction (RT-PCR)2.

The post is correct in affirming that mouth swabs are a highly effective collection method for DNA testing. However, such swabs are used to collect cheek cells, not saliva as suggested. Saliva collection does not require a swab—patients are simply asked to spit into a tube. This method is easier to perform and less uncomfortable than the nasopharyngeal swab and is therefore being evaluated as an alternative approach to nasopharyngeal swabs. This method could also reduce the risk of infection for healthcare workers and the need for personal protective equipment and swabs since patients can “collect” the samples themselves. However, several studies, including a meta-analysis (joint analysis of data from several independent studies), suggest that the viral load in saliva is lower than in nasopharyngeal samples and highly variable between patients3-6. Furthermore, the virus is detected only during the early phase of the infection and then declines rapidly over time7. Hence, saliva testing may be less accurate and could miss a proportion of COVID-19 cases compared to nasopharyngeal testing.

Even though saliva is not a preferred specimen in COVID-19 diagnostic testing, it may be complementary to nasopharyngeal swab testing and increase the chances of accurately diagnosing infections. In April 2020, the U.S. Food and Drug Administration (FDA) authorized the emergency use of the first at-home collection kit for COVID-19 testing using nasal swabs. Since then, the FDA has approved several other at-home collection devices for nasal mucous samples (Everlywell and KPMAS kits) and saliva samples (Rutgers, Phosphorus, and LetsGetChecked kits). While experts recognize the merits of these kits in terms of comfort and safety, they have also raised serious concerns about their reliability due to potential errors in at-home sample collection.

In a letter to FDA Commissioner Stephen Hahn, the American Association for Clinical Chemistry (AACC) recommended that the FDA issue a warning about the accuracy of COVID-19 at-home tests. The AACC is a global scientific and medical organization providing insight and guidance in the field of clinical laboratory science and its application to healthcare. According to the AACC, “problems commonly occur with self-collection that can affect the quality of the sample and, therefore, the subsequent test result”. Major concerns involve sample quantity, sterility, incorrect handling, or viral integrity during shipping. False-negative results could impact public health since infected individuals may “fail to be treated properly and take appropriate actions to prevent the spread of the disease”.

The claim that “a droplet of saliva will infect a whole village at one time” is inaccurate. Although the specific infective capacity of saliva and its role in transmission are not clear yet8, this claim overestimates the infectivity of a single drop. The highest risk of contagion has primarily been reported for people sharing a confined space for long periods of time and releasing thousands of respiratory droplets9. As previously explained by Health Feedback, respiratory droplets are small particles of mucus or saliva that a person releases when coughing, sneezing, or talking and are considered the main mode of COVID-19 transmission.

In contrast, droplets smaller than 5 micrometers in diameter can form aerosols which float farther away than respiratory droplets, becoming an alternative transmission pathway under certain circumstances. On 6 July, 250 scientists issued a letter calling on the World Health Organization (WHO) to consider airborne transmission as a significant mode of disease transmission for COVID-19. This led the WHO to acknowledge “the possibility of aerosol transmission, combined with droplet transmission, particularly in indoor settings with poor ventilation”.

In summary, based on current data, nasopharyngeal swabs are the most reliable method for COVID-19 diagnostic testing so far. The relative diagnostic capacity of saliva testing appears inferior, at least until precise collection and detection methods are specifically optimized for saliva. In the meantime, saliva testing may be a useful complementary approach to help increase testing capacity in circumstances when swabs are in short supply or difficult to use, like in children.



Published on: 14 Jul 2020 | Editor:

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