Covid-19 saliva testing strategy could mean easy-to-use home device, experts say

Nina Massey, PA Science Correspondent
·3-min read

A new two-stage coronavirus saliva testing strategy that is simpler and easier to use than nasal swab tests has been developed by scientists.

In the first stage, the virus in saliva is amplified using an isothermal (constant temperature) reaction to give a positive or negative result for Sars-CoV-2 within two hours.

The second stage involves genetic sequencing of the sample to verify the results.

This two-stage process would reduce the risk of potential false positive or false negative results, and could potentially be scaled up to facilitate population-level testing, researchers say.

The test has been developed by scientists at the Wellcome Sanger Institute and their collaborators.

Chenqu Suo, joint first author of the study from the Wellcome Sanger Institute, said: “Using saliva as an input, and an isothermal reaction to amplify viral RNA, has the advantage of simplifying the Covid-19 testing process considerably, while maintaining ‘gold standard’ accuracy.

“Firstly, this raises the possibility of scaling up centralised testing in laboratories.

“Secondly, it means a cheap, portable device for use in the home could become a reality in future.”

The most widely-used testing strategy for coronavirus in the UK currently requires a nasal swab to be taken from deep in the mucous membrane.

Swabs are then sent to a laboratory to be processed by machine which amplifies any viral RNA present to detectable levels.

The laboratory process takes around four hours and is conducted on machines that are not portable.

However, experts say taking a nasal swab can be uncomfortable and difficult to do correctly, especially for those who are self-testing.

Difficulties in administering the test can also lead to inaccurate results.

In findings published in Science Advances, samples of synthetic viral RNA mixed with saliva from healthy individuals were subjected to an isothermal NASBA reaction.

Nucleic Acid Sequence-Based Amplification (NASBA) is a way of amplifying RNA of interest to detectable levels, using a combination of chemicals and a constant temperature (isothermal).

The use of an isothermal NASBA reaction allows RNA amplification to be achieved in any portable device that can maintain a constant temperature of 41C.

Fluorescence detection, as well as a home pregnancy-style dipstick reading, were then used to consistently detect the presence of around 50 copies of SARS-CoV-2 viral RNA within two hours.

The researchers say this sensitivity is comparable to the current test used in the UK’s public testing strategy.

The second step in the new testing strategy, as well as helping to reduce the risk of home testing errors, could also be used independently as a sequencing-based mass testing strategy.

When someone completes the test, a DNA barcode is added to any virus present in the saliva, allowing the sample to be identified at a later point.

The sealed sample would then be posted to a sequencing facility, where tens of thousands of samples could be sequenced simultaneously in around 12 hours to confirm whether the virus is present or not.

Current tests are processed in batches of up to 384 samples that take around four hours to complete and involve several manual laboratory steps.

Dr Qianxin Wu, joint first author of the study from the Wellcome Sanger Institute, said: “Any effective testing strategy requires confidence in its reliability, but must also be scalable.

“We believe DNA barcoding combined with a highly-multiplexed sequencing would be a feasible population scale Covid-19 testing strategy.”

The next phase of the project will be further research and development using patient samples, which is currently underway.

Researchers hope the equipment required to perform the test could be incorporated into a portable device for use by non-experts at home.