We all realise that testing for COVID is important for public health, medicine and science – but what does it mean for you? The different tests available can be confusing, and you may not be clear on why these tests are being performed and what the implications (and limitations) of their results are.

Here, we take a more detailed look at what the different types of tests available can and can’t tell us about COVID-19 and what that means not only for you but society as a whole. It is really important that everyone understands COVID testing so everyone feels informed about being tested and has confidence in their test results.

How can I find out if I have COVID?

There are two tests available currently, that can determine if someone is infected with the SARS-CoV-2 virus that causes the illness COVID-19. These tests will look for the presence of the virus inside your body.

The most common test used is the PCR Test. This test can identify the presence of genetic materials from the virus. The other test, the antigen test, is used to identify the presence of certain proteins from the virus. These two tests are often confused, and their names tend to be used interchangeably – which is incorrect. They are two very different and distinct tests.

To explain it simply, the Medicines & Healthcare Products Regulatory Agency (MHRA), which is the UK’s regulator of medicine, uses the PCR test to determine whether someone is currently infected with the SARS-CoV-2 virus. So, if you have symptoms of COVID and request a test from the NHS to see if you do have COVID-19, they will use a PCR test to determine this.

In August last year (2020) the UK government announced that they would be using two rapid COVID-19 tests ‘on the spot’ to diagnose COVID-19 infections in care homes and hospitals. These tests do not need a trained professional to be administered, and results can be generated within 90 minutes. Only one of the two tests they used has been approved by the MHRA.

A study entitled “Assessing a novel, lab-free, point-of-care test for SARS-CoV-2: a diagnostic accuracy study”. published in September 2020 found that the accuracy of these tests were comparable to the PCR tests, meaning the rapid test used in care homes can correctly identify SARS-CoV-2 genetic material as well as a laboratory PCR test can.

How does the PCR test work?

The PCR test requires a swab sample to be taken from the nose and the throat of the patient, and then this should be sent to a laboratory for analysis. The swab can be taken by the person themselves or someone else, at home or at a drive-through testing centre.

Samples need to be taken within a week or so after a person has been infected, even if they are showing no symptoms. Samples will only come back as positive if a person is actively infected with the COVID virus. Once the virus infection has cleared, the test can’t be used as it is unable to tell you if you have previously been infected or not as there is no viral genetic material left in your body.

A PCR, or polymerase chain reaction, is the name of the laboratory technique used to detect small amounts of genetic material from a sample. If the sample contains enough viral genetic material that it can be amplified above a certain level, then a person is deemed to be ‘positive’ for the virus. The PCR test tells us if a person has the virus at the time that the sample was taken.

PCR is a long-established scientific technique that has a high accuracy rate (when performed by trained laboratory technicians). It is a very sensitive test that is used to detect very low levels of genetic material from a sample – as long as the swab sampling of the nose and throat is done correctly, and at the appropriate time to ensure there is enough of the virus present on the swab.

Tests can sometimes give false-negative results, between 2% and 29% of the time, meaning a true infection can sometimes be missed as no viral genetic material is detected. The person who was tested may then think they do not have COVID-19, when in fact they do, meaning they could unknowingly spread it to others.

Image downloaded British Society for Immunology

Why is it important to know who has COVID-19?

Detecting who is currently infected with COVID-19 is important in identifying the people who need to isolate to help contain the spread of the infection. If you know that you have the virus then you can adapt your behaviour, self-isolate and prevent the virus from being passed to other people. This knowledge can also be used to contact exposed individuals so they can also be vigilant and adapt their behaviour.

The information gained from testing is also important for the UK government to be able to monitor how many people are infected, allowing them to make more informed decisions about public health in order to contain the outbreak. As the pandemic has progressed PCR testing has helped indicate how the virus has spread through the UK population over time, how it affects various groups of people and the risk factors that are potentially associated with a worse outcome in infection.

Can testing be used to tell if I have had COVID in the past?

When a person becomes infected with SARS-CoV-2, specialised cells in the immune system called B cells, produce antibodies to fight the infection. Antibodies are proteins that precisely match and recognise the antigens of the virus. An antigen is a unique feature of the virus which induces an immune response in the body and is usually seen as a protein on the outside surface.

Antibodies stick to specific antigens and mark the virus for destruction by other cells of the immune system, thereby helping the body to clear the infection. Antibodies can also prevent the virus from attaching to cells – such as those in the lung lining – which then limits the spread of the infection within our bodies. These antibodies, which prevent the virus from infecting our cells, are called neutralising antibodies.

After a person has been exposed to the virus, even after the infection has been cleared, these antibodies will stay in the body for some time – although their levels will decrease over time. Therefore, if you have not been infected with COVID you won’t have the antibodies against it in your body.

What can antibody testing tell us?

Antibody testing requires the use of a blood sample, which is then tested to see if there are any antibodies present that binds to a specific viral antigen. Blood contains serum, which is the part of the blood that does not contain red or white blood cells or clotting factors. The antibody test uses this blood serum and is called a serology test.

B cells need time to mount an appropriate response to a virus and produce new antibodies, and so antibodies will only reach detectable levels in the blood around a week after you have been initially infected. The presence of antibodies in your blood is a reliable indication that your immune system has responded to SARS-CoV-2 in the past and started an immune response to it.

Antibody testing is currently being used to assess how many people have been previously infected and to understand how the virus is spreading. Studying this in public health settings is most helpful for preventing disease in the general UK population and containing the spread of the virus.

In the longer term, antibody testing may be able to tell us how long antibodies stay in our system after the infection has cleared. This information will be useful in determining how effective COVID-19 vaccines are.

What does antibody testing not tell us?

The presence of antibodies within a person does not tell us that a person is immune and therefore protected from future infection. Studies are still ongoing to assess if the antibodies produced after SARS-CoV-2 infection do prevent you from catching the virus again or not, and how long-lasting this protection might be.

It is still not clear whether everyone who has had COVID-19 will have the antibodies for it in their blood. Protection against the virus may also depend on other parts of the immune system, such as the response of white blood cells (called T cells) and these are not measured by the current antibody test. Therefore more research is needed to understand COVID immunity.

On top of this, antibody tests must be carefully used as false-negative results can occur if a person is tested too early due to having detectable antibodies in their blood, even though they have had COVID-19. Therefore the time since the onset of symptoms began is important for deciding when the rest should be performed.

How accurate are antibody tests?

In order for a test to be considered accurate, it must be specific and sensitive to SARS-CoV-2 antibodies. Sensitivity means that the test can currently identify those who have antibodies (true positives) while specificity relates to the ability of the test to correctly identify those who do not have antibodies (true negatives).

If tests are not highly accurate, then they risk giving someone a false positive result, as they can wrongly suggest that people have not been previously exposed to the COVID virus, when in fact they have. It is also important that tests only identify SARS-CoV-2 antibodies, meaning the antigen used in the test has to be unique to SARS-CoV-2 and not found in other human coronaviruses such as the common cold.

What does all this mean for you?

A PCR test is used to determine if you are currently infected with SARS-CoV-2 (COVID-19) – which is why many countries are now requesting you provide them with a negative PCR test result before you are allowed entry.

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