Unfettered access to rapid and safe Covid-19 tests will be one of the most critical factors in reducing the spread of the disease. Governments all around the globe are hoping to gain access to an assay which can be deployed effectively and swiftly throughout the healthcare system, and home testing is likely to play a key role in this deployment.

There’s an ever-growing catalogue of Covid-19 tests out there, and if a test is given out to large swathes of the population then it’s vital that it can stand up to scrutiny. Even then, mass home testing isn’t without its risks.

“These are tests that are being developed very quickly, so it’s key to understand how accurate they are in terms of their sensitivity and specificity before we can really gauge how reliable the results would be,” says University of Sussex senior lecturer in microbiology Dr Edward Wright.

Backwards directly into the face

So far, the tests which have dominated the discussion around screening for Covid-19 have been polymerise chain reaction (PCR) tests. Considering their current market dominance, it’s difficult for PCR test to be effectively repurposed for home use.

PCR tests are a type of antigen test, meaning they work by tracing the SARS-CoV-2 virus which causes Covid-19 directly. They do this through swabs taken deep inside the nasal cavity, and have to undergo specialist analysis in a laboratory to yield results.

“To do a PCR test properly you actually need to go 5 – 10 cm backwards directly into the face. It looks quite horrific. People don’t think that your nasal passages are very deep, but the swab needs to be in the right place to get the greatest probability of getting a good sample,” says Warwick Medical School honorary clinical lecturer Dr James Gill. “So we’ve got to assume that the patient swabs themselves correctly, puts it back in the pot correctly and it doesn’t get damaged in the post, all before it gets analysed in the lab.”

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Covid-19 is traceable via an antigen test like this as soon as infection is established in the body, before symptoms or antibodies have time to develop. This can allow infected people to self-isolate earlier, meaning PCR tests will continue to have a crucial role to play screening for the virus in clinics.

Laypeople at home, however, will need a test that’s easier to administer.

Mapping the population

Typically, a viral infection is followed by an immunity period to the disease. It’s currently unknown exactly how long this might be for Covid-19, but similar coronaviruses like sudden acute respiratory syndrome (SARS) can trigger an immunity period that lasts up to several years. Researchers are hoping that Covid-19 has a similar effect, and that once a person has the infection and recovers it will be safe for them to re-enter society as they will no longer be able to contract or spread the disease.

Antibody tests seek out this immune response, rather than trying to directly detect the antigen itself. They’re not the best at indicating who’s actively infected, as antibodies can take a few weeks to form, but they’re invaluable in mapping the spread of the disease and herd immunity throughout a given population. If somebody tests positive Covid-19 antigens, then they can be reliably assumed to have contracted and recovered from the disease.

Crucially, antibody tests are also much easier to self-administer than PCR tests. Many use blood samples, which can be collected at home with a simple finger-prick device. These kinds of samples are already sent in the post fairly regularly, as part of home HIV and syphilis test kits distributed by sexual health clinics.

Other Covid-19 home antibody tests in development describe a ‘pregnancy test’ style device the blood sample can be distributed into, which will provide a result within a few minutes and eliminate the need for lab testing.

Testing for active and historic infections

Right now, it seems that there’s no watertight option. PCR tests are too difficult to self-administer and while using an antibody test is more straightforward, it won’t be as effective at detecting early stage infections.

Gill says: “The best way of confirming you have the virus is running both a PCR and an antibody test. Then you get a 98.6% true reading, i.e. if you’ve got it, you’ve definitely got it.”

The best route for mass home testing may therefore be to distribute some kind of home-appropriate antigen test alongside an antibody test, allowing both active and historic infections to be established simultaneously throughout the population.

“The more information we have fed back the clearer the picture is of who has been infected, and the better our understanding going forward,” says Wright. “We get a more accurate idea of what would then happen if certain strategies or interventions were put in place.”

Covid-19, while tied most commonly to a dry cough, fever and shortness of breath, has a nebulous range of secondary symptoms which have left many people unsure of what’s been ailing them. Providing both kinds of test could help people understand their status in relation to both active and historic infection.

Many firms have gotten to work on developing self-administrable antigen tests, the first of which are beginning to receive Emergency Use Authorisation (EUA) from the US Food and Drug Administration (FDA). Crucially, the agency has reissued the EUA for LabCorp’s COVID-19 RT-PCR Test, to permit the testing of samples self-collected by patients at home using LabCorp’s Pixel by LabCorp COVID-19 Test home collection kit.

The reissued EUA will allow the testing of a samples collected from the front of a patient’s nasal cavity with the Pixel kit, instead of the deep swab used in a routine PCR test. The at-home version of the test must be recommended by a healthcare provider, with its $119 price tag covering kit materials, overnight shipping and the costs of actually testing the sample. Currently, a limited number of kits are being made available to healthcare workers and first responders who may have been exposed to Covid-19 or are experiencing symptoms, with eventual plans to make the kits available to consumers in most states.

The FDA has cautioned that: “This authorisation only applies to the LabCorp COVID-19 RT-PCR Test for at-home collection of nasal swab specimens using the Pixel by LabCorp COVID-19 home collection kit. It is important to note that this is not a general authorization for at-home collection of patient samples using other collection swabs, media, or tests, or for tests fully conducted at home.”

OraSure Technologies, on the other hand, is looking to repurpose the platform used to build its rapid HIV self-test to screen for Covid-19 at home using a gum swab. This test, which would seek out SARS-CoV-2 directly like a PCR, could help people confirm active infections from home before antibodies develop. Alongside this, OraSure is working on developing a non-rapid laboratory antibody test, which uses oral fluid to detect the immune response to the disease.

“There is clearly value in having tests that detect the antigen and the antibodies,” says OraSure CEO Stephen Tang. “I think the most acute issue right now is getting those who are infected and infectious to stay at home. Allowing people to understand their own disease status will give them control over their lives.”