How do antibody tests for coronavirus work?

The new antibody tests which Public Health England is hoping to roll out have been billed as a “game changer” in fighting coronavirus. Such a test will allow the government and health authorities to discover who has been infected with the SARS-CoV-2 virus and recovered, whereas current tests can only indicate whether someone has the virus right now.

The potential benefits of such a test in the fight against COVID-19, if it can be proven to be accurate, are wide ranging both in the short-term and long-term. I have summarised these benefits elsewhere.

So how exactly do these tests work?

The antibody test, as the name suggests, detects antibodies for the SARS-CoV-2 virus. It is a serological test, which means that it is based upon testing a person’s blood. They will require a pin prick of blood from someone’s finger which will then be used by the test to identify whether or not a person has antibodies to the coronavirus.

This works because the test kit itself is what is known as an “immunoassay”. This means that it is a biochemical test which can identify viral exposure and the presence of antibodies.

Antibodies provide the body’s immune response to coronavirus. If someone has immunity to coronavirus, the test kit will change colour to indicate the presence of antibodies.

This happens because the antibodies in the blood are used to targeting a specific “spike” protein on the outer coat of the SARS-CoV-2 virus. This protein helps the virus enter the body’s cells and is a key target of antibodies.

The developers of antibody test kits design a slightly altered version of this spike protein and reproduce large numbers of it to be used in the diagnostic antibody test. When a sample of a person’s blood or plasma is tested, the antibodies in it identify the target protein in the test as if it were that of the virus, bind to it, and trigger the colour change on the test kit.

Bringing antibody tests into play would enable the health authorities to adopt more effective testing processes as the epidemic progresses.

Currently, the PCR tests which are being used in the UK– which detect specific genetic material within the virus – are being conducted centrally at hospitals and testing centres. They are carried out by taking a swab sample and sending it off to a laboratory. This requires transportation time and in the UK it is believed to take around 24 hours to get a result back.

Immunoassays are less reliant upon limited lab capacity. They will be able to facilitate decentralised point-of-care testing – remote testing in the community, in GP surgeries and in our own homes and living rooms. They will also be quicker, with some prototypes providing a rapid result in only 20 minutes.

Results could then be sent in to epidemiologists compiling data to help them identify where resources need to be concentrated to tackle the coronavirus. Tim Colbourn, Associate Professor of Global Health Epidemiology and Evaluation at UCL Institute for Global Health, told Reaction that an ideal situation would be if “everyone got a Coronavirus test kit posted through their letter box.”

“Then the whole country could know whether they have it or not. If the test results were sent back on an app and stored with geolocation data epidemiologists and public health authorities will know exactly where every case was and be able to intervene to stop spread.”

And by reversing the process and designing an antigen test – which uses monoclonal antibodies to identify the presence of viral antigens – it is also possible to develop an immunoassay to test for the virus as well.

Still, with both the antigen and antibody tests, particular caution is needed. There is a possibility that the diagnostic could pick up different types of antibodies which are used to targeting the same or similar proteins on other viruses. Potential pitfalls like this need to be ironed out in validation tests before such diagnostics can be rolled out on a large scale with confidence.

These new tests – if they could be validated as being accurate – would allow for a much better understanding of how many people have had the coronavirus. It would also help provide a better indication of where the virus has spread and how fast, as well as who is still susceptible to infection.