University of Hong Kong: Switching Up the Swab Test

Image credit: “State Public Health Laboratory in Exton Tests for COVID-19” by governortomwolf is under a CC BY 2.0 license.

The paper that we’ll be demystifying can be found here, if you want to follow along.

Our main test for COVID-19 is the swab test, where a long swab is used to take a sample from the nasal passages. In certain cases, swabs from the throat are also used to check for SARS-CoV-2.

Now, it’s important to note that while this article brings up potential risks of these tests, if you do suspect you have COVID-19, please get tested regardless.

Swabbing the nasal passages and throat can make a patient sneeze or cough, putting healthcare professionals at risk of infection. Additionally, the nasal swab can be fairly invasive and uncomfortable, in rare cases causing slight bleeding. This discomfort can cause some COVID-19 patients to shy away from testing, putting the community at risk.

Thankfully, a team of researchers from the University of Hong Kong and doctors from Princess Margaret, Queen Mary, and Queen Elizabeth Hospitals have been studying alternate ways to get workable samples for testing. Focusing on safer methods, they explored testing saliva and serum, the liquid component of blood.

Saliva samples were taken by coughing up a sample first thing in the morning, before they had brushed their teeth or eaten breakfast, as to limit contamination. A sample right after waking up is also ideal because the body naturally pushes lung secretion up to the back of the throat during sleep. To test for the presence of viruses in the saliva sample, the team used quantitative RT-PCR, targeting the Rdrp/Hel element of the SARS-CoV-2 genome, a technique explained in a previous post.

The average number of viruses in patient saliva, or viral load, was around 158,000 copies per mL. The load was highest in the first week of COVID-19 symptoms, steadily declining afterwards. 7 of the 21 patients studied had SARS-CoV-2 traces detected 20 days after symptoms began or more, with one even having traces 25 days after symptoms started. This suggests that unlike severe acute respiratory syndrome, COVID-19 patients have the greatest viral load around the presentation of symptoms. As this would be during the coughing period, this might partially explain why COVID-19 has spread so quicly.

For intubated patients who therefore couldn’t provide saliva samples, researchers checked the endotracheal aspirate, the secretions that collect during some intubations. The viral load in these samples didn’t seem to decline significantly during intubation.

Viral load also seemed to increase with age. There was no relationship between the length of time that the viral RNA could be detected in saliva and the severity of the disease.  

For the serum samples, the team looked at the levels of specific antibodies in the blood. Antibodies are Y-shaped molecules created by the immune system to neutralize invaders, such as SARS-CoV-2. An antibody is specific to a particular type of virus, even specific to a component of a virus. During a viral infection, the antibody that binds to the virus in question is proliferated, essentially multiplied in the bloodstream.

COVID-19 infection is no exception. Various antibodies specific to different parts of SARS-CoV-2 circulate the bloodstream in elevated amounts. The team in Hong Kong looked at antibodies that act on the SARS-CoV-2 nucleoprotein (NP), proteins associated with the virus’ genetic material, and receptor binding domain (RBD), which allows entry into human cells.

Researchers used two tests to measure the presence of anti-NP and anti-RBD antibodies. The first was an enzyme immunoassay (EIA), in which isolated SARS-CoV-2 NP and RBD were exposed to serum samples. Any binding between the SARS-CoV-2 fragments and antibodies, which caused a proportional color change, represented the presence of these specific antibodies.

The second test was a microneutralization assay using cultures of SARS-CoV-2. Using the same principle as the EIA, this test measured the amount of antibody activity on the entire virus, which maintained its infectious capabilities.

From both tests, researchers found that most patients tested positive for anti-NP and anti-RBD antibodies. Additionally, like with the saliva samples, severity of the COVID-19 infection had no effect on the levels of anti-RBD and anti-NP antibodies in the serum.

It’s important to note that we’re focusing specifically on the team’s experiments. The paper has lots of extra interpretations of their results that you can check out here.

The team’s work has opened up two promising new methods for COVID-19 testing, serum samples and early-morning saliva samples. The saliva route is especially valuable, as it’s safer for health-care workers and less invasive, which should incentivize testing as a whole.

Until then, getting swab tested is still far better than no test at all. Don’t hesitate, help keep everyone safe.

Paper Citation: To K, Tsang O, Leung W et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study. The Lancet Infectious Diseases. 2020. doi:10.1016/s1473-3099(20)30196-1