Testing for COVID-19 can be informative and important for science, medicine and public health but what does it mean for you? The different tests can be confusing and sometimes it’s not clear why these tests are being performed and what the implications and limitations of their results truly are. Here, we take a detailed look at what the different types of available tests can and can’t tell us about COVID-19 and what that means for you as an individual and society as a whole. It’s important to strengthen understanding about testing so everyone feels informed about being tested and has confidence in test results.
How do I know if I have COVID-19?
There are two types of test that can determine if someone is currently infected with SARS-CoV-2, the virus that causes the illness, COVID-19. These tests look for the presence of the virus inside your body. The PCR test identifies the presence of genetic material from the virus, whilst the antigen test identifies the presence of specific proteins from the virus. Sometimes, these two tests are confused, and their names are incorrectly used interchangeably. However, they are two distinct and different tests. The UK’s regulator of medicines, the Medicines & Healthcare products Regulatory Agency (MHRA), uses the PCR test to determine if someone has a current infection with SARS-CoV-2. If you have symptoms and request a test from the NHS to find out if you have COVID-19, the PCR test will be used.
In August 2020, the UK Government announced that two new rapid COVID-19 tests, which detect SARS-CoV-2 genetic material in 90 minutes, will be used ‘on the spot’ to diagnose COVID-19 in hospitals and care homes. The announcement states that they do not require a trained professional to perform the test or a laboratory setting, instead using patient nose swab or saliva samples with table-top portable machines. These two rapid tests are being evaluated whilst rolling out use across the NHS. Only one of these tests, the CovidNudge test, has been approved by the MHRA. A study published in September 2020 found that the diagnostic accuracy of the CovidNudge test is comparable to the PCR test. This means that the CovidNudge rapid test can correctly identify SARS-CoV-2 genetic material as well as the laboratory PCR test.
How does a PCR test work?
The PCR test requires a swab sample to be taken from the nose and throat of the patient, which is then sent to a laboratory for analysis. The swab can be taken by the person themselves or someone else (usually a healthcare professional), at home or at a drive-through centre. Samples need to be taken within a week or so after a person has been infected, even if they have no symptoms. Samples are only useful during active infection with coronavirus. Once the infection has cleared, the test cannot be used and will not tell you if you have been infected or not because the viral genetic material is no longer present in your body.
A polymerase chain reaction (PCR) is a laboratory technique used to detect small amounts of genetic material from a sample. If there is enough viral genetic material within an individual’s sample that can be amplified above a set level, that person is deemed to be “positive” for the virus. The test tells us if a person has the infection at the time when the swab sample was taken.
PCR is an established scientific technique and has a high accuracy rate when performed by trained laboratory professionals. It is a very sensitive test that can detect very low levels of genetic material from a sample. However, the swab sampling of the nose and throat needs to be done correctly and at the appropriate time after infection to ensure there’s enough of the virus on the swab. Tests can give false negative results, which can occur between 2% and 29% of the time. The risk of false negatives means a true infection is missed because no viral genetic material is detected. The patient may then think that they do not have COVID-19 but can risk spreading it to others. Repeat sampling and testing may be important to rightly diagnose patients who have symptoms of COVID-19 and are likely to have been exposed to the virus. Due to the difficulties in taking good samples from the nose and throat, there is interest in developing a test that can detect virus from saliva. This type of test is currently being assessed.
Why is it important to know who has COVID-19?
Detecting who currently has a SARS-CoV-2 infection is important to identify people who need to be isolated to contain the spread of infection. If you know you have COVID-19 you should adapt your behaviour, self-isolate and prevent spreading it to others. This knowledge is also used for contact tracing of exposed individuals so they can also be vigilant and adapt their behaviour.
Testing information is also important for monitoring how many people are infected and can be useful for making public health decisions about measures to contain the outbreak. As the pandemic progresses, PCR testing indicates how the virus is spreading through the population over time. Importantly, this data can reveal how the virus affects different groups of people and can identify risk factors (e.g. age, gender, etc.) associated with a worse outcome in infection.
Can I tell if I’ve had COVID-19 in the past?
When someone becomes infected with SARS-CoV-2, specialised cells of the immune system called B cells produce antibodies to fight the infection. Antibodies are proteins that precisely match and recognise antigens of the new coronavirus. An antigen is a unique feature of the virus which induces an immune response in the body and is usually a protein on the outside surface. Antibodies stick to the specific antigen and mark the virus for destruction by other cells of the immune system thereby helping the body to clear the infection. Antibodies can also block the virus from attaching to cells such as those in the lining of the lungs, which limits the spread of infection in the body. These antibodies which prevent the virus from infecting our cells are called neutralising antibodies. After exposure to the virus, even after the infection has been cleared, these specific antibodies remain in the body for some time, although levels gradually decrease over time. If you have not been infected with SARS-CoV-2, you will not have antibodies against it in your body. Therefore, testing for the presence of antibodies can reveal whether the person has had COVID-19 recently.
What can antibody testing tell us?
Antibody testing requires a blood sample, which is used to test if there are any antibodies present that bind to a specific viral antigen. This is done as a laboratory test using a blood sample. Blood contains serum, which is the part of blood without red or white blood cells or clotting factors. The antibody test uses blood serum and is called a serology test. B cells need time to mount an appropriate response and produce new antibodies to SARS-CoV-2, so antibodies will only reach detectable levels in the blood around a week after initial infection. The presence of antibodies is a reliable indication that your immune system has responded to SARS-CoV-2 in the past and mounted an immune response against it.
Currently, antibody testing is used to assess how many people have been infected previously and to understand how the virus has spread. Studying this is most helpful in public health settings and for informing policy measures to contain the spread of the virus and prevent disease in the population.
In the longer term, antibody testing may tell us how long antibodies persist after infection. This information is helpful when developing vaccines against COVID-19 to measure how effective the vaccine is. This research is ongoing.
What does antibody testing not tell us?
The presence of antibodies does not necessarily mean a person is immune and protected from future infection of SARS-CoV-2. Studies are currently ongoing to assess if the antibodies produced after SARS-CoV-2 infection prevent you catching the disease again and how long-lasting any protection may be.
It is not yet clear whether everyone who had COVID-19 has antibodies to the disease in their blood. However, protection may also depend on other parts of the immune system, including the response of white blood cells called T cells, which are not measured by this test. It is still early days when it comes to understanding immunity for COVID-19 and more research is needed.
Furthermore, antibody tests must be used carefully. False negative results can occur if a person is tested too early before they have detectable antibodies in their blood even though they’ve had COVID-19. Therefore, the time since onset of symptoms is important for deciding when the test should be performed. Results from antibody tests should not guide an individual’s behaviour.
How accurate are antibody tests?
To be accurate, an antibody test must be specific and sensitive to detecting SARS-CoV-2 antibodies. Sensitivity is the ability of the test to correctly identify those who have antibodies (true positives), whilst specificity is the ability of the test to correctly identify those who do not have antibodies (true negatives). If tests are not highly accurate, they risk giving false positive results which wrongly suggest people have previously been exposed to the new coronavirus when they haven’t, or false negative results, which wrongly suggest people have not been previously exposed to SARS-CoV-2 when they have. It’s also important that the test only identifies SARS-CoV-2 antibodies, which means that the antigen used in the test has to be unique to SARS-CoV-2 and not found in other known human coronaviruses, for example those which cause common colds.
Two antibody laboratory tests are now being used in the UK after evaluation by Public Health England (PHE). The overall specificity of these tests was evaluated to be 100%, meaning it is very unlikely that the tests will identify false positive results. The overall sensitivity was 84% and 93%, which means that there is a possibility of false negative results.
What about home testing for antibodies?
Firstly, it’s important to distinguish between home testing and home sampling. Home testing kits refer to a test that you conduct yourself and read the result at home (like a home pregnancy test) but currently there are no COVID-19 antibody testing kits certified for home use in the UK. More research is needed to assess accuracy, ease of use and acceptability of home testing before it can be widely used.
A home sampling kit refers to collecting a sample at home, such as a small blood sample from a finger prick, and sending the sample to an approved laboratory for analysis. Two companies announced they would be selling COVID-19 antibody home sampling kits in the UK but the MHRA quickly stepped in to suspend these services whilst validation takes place. There are uncertainties around how compatible home sampling is with laboratory serology tests. There is currently not enough research comparing blood taken from a fingertip at home to blood taken from a vein by a professional to know how accurate the results are.
The main concern of home testing is interpreting the result and understanding the test’s limitations. For example, someone with a false positive result may incorrectly assume that they have immunity to SARS-CoV-2 and take unnecessary risks. Or someone with a true positive result may not understand that it does not necessarily mean they are immune and could place themselves at risk of re-infection.
What does this all mean for me?
A PCR test is used to determine if you are currently infected with SARS-CoV-2 and this test will be used if you have symptoms of COVID-19 and request a test from the NHS. Antibody testing will tell you if you have previously been exposed to SARS-CoV-2 but it may not necessarily tell you about your long-term immunity to COVID-19. Together these tests give complementary information that can help us learn about the virus and support public health measures.
It’s important to understand the complexities of testing so you can make informed decision about your health.
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We've created new infographics about COVID-19 testing to help explain the important concepts discussed in this blog. They are for everyone to use and share to aid understanding on the important topic.
Public Engagement Officer, BSI