This blog was originally published in July 2020 but knowledge of and guidance on COVID-19 testing has evolved and this blog has been updated in January 2022 to best reflect current guidelines on testing in the UK.
Testing for COVID-19 can be informative and important for 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 for our 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 tests 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, commonly known as the rapid lateral flow test in the UK, identifies the presence of specific proteins from the virus. These are two distinct and different tests. If you have symptoms and request a test from the NHS to find out if you have COVID-19, you will be asked to take a PCR test and the results will be analysed in a laboratory. A PCR test is the most accurate test we currently have available to determine if someone is infected with SARS-CoV-2.
If you do not have symptoms of COVID-19, you can take a rapid lateral flow test using a hand-held device. A lateral flow test is used at home or in the workplace and can indicate if you are likely to be carrying the virus and infectious at the time you take it, with results available within 15–30 minutes. The UK Government and devolved administrations now advise regular use of lateral flow tests, for example twice per week or before close contact with other people. A regular testing programme was also rolled out for use by healthcare workers to prevent the spread of the virus in hospital and care settings. The reason to do this is to detect infection before people develop symptoms or to detect individuals who might carry the virus without symptoms.
How does a PCR test work and what can it tell us?
The PCR test requires a swab sample to be taken from the nose and throat of the person, which is then sent to a laboratory for analysis. The swab can be performed by the person themselves or someone else (usually a healthcare professional), at home or at a designated drive-through or walk-through centre. Samples are only useful during active infection with SARS-CoV-2, but a PCR test can detect if the virus is present even before symptoms appear and can also detect asymptomatic infections (infections without symptoms). In most people, once the infection has cleared, the PCR test can’t tell you if you have been infected or not because the viral genetic material is no longer present in your body. For some people, small fragments of the virus can be persistently detected by a PCR test for some time after an infection, even after the person has completed their isolation period and is no longer infectious. Therefore, in the UK, you should not take a PCR test if you've tested positive for COVID-19 in the last 90 days, unless you develop new symptoms.
PCR stands for 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 analysed by trained laboratory professionals in an accredited laboratory. It is a very sensitive test that can detect very low levels of viral genetic material from a sample.
However, the initial swab sampling needs to be done correctly and at the appropriate time during infection to ensure that there’s enough of the virus present in the nose and throat to be picked up on the swab sample. Incorrect sampling techniques, sampling too soon after exposure to the virus or too late after symptom onset could generate a false negative result. The risk of false negatives means a true infection is missed because no viral genetic material is detected. The person 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 the virus from saliva. Initial results look promising, but this test is not widely available yet.
How does a rapid lateral flow (antigen) test work?
A lateral flow test also uses an established scientific technique and tests for specific proteins called antigens. Lateral flow tests are available for use at home and in workplaces to detect proteins from SARS-CoV-2 in someone currently infected with the virus. Test kits provide the individual with everything they need to take a swab sample from the nose and/or throat and to run the test using a hand-held device. There is no need to send the swab sample to a laboratory and you can get results quickly wherever you are. Results can take 15–30 minutes to show on the device. A positive result indicates that proteins from SARS-CoV-2 are present in the sample, meaning that it is likely that the person is currently infected with COVID-19 and must follow the latest government guidelines on isolation.
What can rapid lateral flow tests tell us?
Lateral flow tests are currently encouraged for regular use at home and work to monitor whether people are infected and to detect asymptomatic cases of COVID-19. Around 1 in 3 people with COVID-19 are asymptomatic, meaning they do not have symptoms, so regular lateral flow testing can help to catch cases where the person may have been unaware and unknowingly spreading the virus on to others. This makes lateral flow tests ideal for widespread use in the community to help contain outbreaks.
Lateral flow tests are not as sensitive as PCR tests. This means that a negative result is possible when someone is in fact carrying the virus but only at a low level. As tests are conducted by people in their own homes or workplaces, it is important that the instructions are followed carefully to ensure that the swab samples are taken properly and that the kits are used correctly.
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 isolate to contain the spread of infection. If you know you have COVID-19, or have any of the symptoms of COVID-19 and are awaiting a test result, you should follow the most up-to-date government and NHS guidance, which may include asking you to self-isolate to prevent spreading it to others. This knowledge is also used for contact tracing of exposed individuals so they can be vigilant and adapt their behaviour.
It is still important to test for COVID-19 even if you have received one, two or even three doses of a COVID-19 vaccine as it is still possible to be infected with the virus. Many studies have shown that vaccination is highly effective at preventing the symptoms of COVID-19, especially the most severe symptoms that can result in hospitalisation or death. However, some people can still be infected and carry the virus despite being vaccinated. This means that someone who has been vaccinated but experiences the symptoms of COVID-19 should still get a PCR test.
Information from the COVID-19 testing programme is also important for monitoring how many people are infected with COVID-19 on a regional and national level. This information can be useful for making public health decisions about measures to prevent the spread of infection. As the pandemic progresses, testing indicates how the virus is spreading through the population over time. Importantly, these data can reveal how the virus affects different groups of people and can identify risk factors (e.g. age, sex, etc.) associated with a worse outcome in infection.
When else should I get tested for COVID-19?
It is recommended that if you have been in close contact with someone who’s tested positive for COVID-19, you should test yourself to check if you’ve been infected. If you’re fully vaccinated or under 18 years and six months old, you should take a lateral flow test every day for seven days. If the lateral flow test is positive, you need to isolate. If you are not fully vaccinated, you need to isolate for 10 full days and take a PCR test.
In the UK, if you have COVID-19, you can stop isolating after 5 full days if you take a lateral flow test on day 5 and at least 24 hours later on day 6 and both tests are negative, and you don’t have a high temperature. If the lateral flow test is positive on day 5, you should wait 24 hours to take another test and only stop isolating after you’ve had two negative tests, 24 hours apart.
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 specifically recognise the virus and mark it for destruction by other cells of the immune system thereby helping the body to clear the infection. 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 built an immune response to SARS-CoV-2, you will not have antibodies against it in your body. Testing for the presence of SARS-CoV-2 specific antibodies can reveal whether the person has had COVID-19 in the recent past.
However, antibody testing can also detect antibodies made by the immune system in response to COVID-19 vaccination. COVID-19 vaccines work by exposing the immune system to a harmless form of SARS-CoV-2 in a controlled way without causing an infection or making you ill. This allows an immune response against the virus to develop, including B cells producing antibodies. If you have been vaccinated, antibody testing may also reveal the presence of antibodies made after vaccination and cannot be distinguished from antibodies made after a past infection.
How does an antibody test work?
Antibodies are found in the blood, so antibody testing requires a blood sample. A laboratory test is then performed on the blood sample to measure the amount of specific antibodies that can recognise the virus. 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 from around 7 to 10 days after initial exposure. The presence of antibodies is a reliable indication that your immune system has responded to SARS-CoV-2 in the past and generated an immune response against it, whether through infection or vaccination.
What can antibody testing tell us?
Currently, antibody testing in the UK is not available for free to everyone but in England, the NHS has recently made antibody testing available and optional two weeks after a positive PCR test result. Antibody testing is used to assess how many people have an immune response to the virus. This may be because they have previously been infected with the virus, or because they have been vaccinated. Studying the number of people with an immune response to SARS-CoV-2 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. Antibody testing is also important for determining local or national infection rates because a large number of individuals are asymptomatic or only have mild symptoms and therefore they may not realise that they should go for PCR testing.
Studies have suggested that antibodies against SARS-CoV-2 last at least several months after infection or vaccination. However, continued longer-term testing is needed to understand how long they may persist in your body. Importantly, antibody levels are highly variable between individuals. While the vast majority of people will develop antibodies against the virus following infection or vaccination, response levels may be lower in those with certain underlying health conditions or weakened immune systems; further study is needed to understand this. Antibody testing can therefore be an important part of research into the immune responses generated by COVID-19 vaccines and infection.
What does antibody testing not tell us?
The presence of antibodies does not necessarily mean a person is immune and protected from future infection with SARS-CoV-2. Studies are currently ongoing to assess how well the antibodies produced after SARS-CoV-2 infection or vaccination protect you from catching the disease again and how long-lasting any protection may be, although this protection is likely to last at least several months after exposure.
It is not yet clear whether everyone who has previously had COVID-19 develops antibodies against the virus in their blood. Studies show that the level of antibodies generated after infection varies between people; however, more research is needed to know how this correlates with protection from COVID-19. Antibodies are only one part of the immune response against COVID-19 and a lack of antibodies doesn’t necessarily mean you can’t fight off the infection. Protection against COVID-19 is also likely to involve other parts of the immune system, including the response of white blood cells called T cells, which are not measured by this test.
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 if they’ve had COVID-19 or they’ve been vaccinated. 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).
The antibody tests currently used in the UK are highly specific, meaning it is very unlikely that the tests will identify false positive results. The test sensitivity is slightly lower, meaning that there is a small possibility of false negative results, although the majority of people with antibodies should still test positive if a detectable level of antibodies is present. The accuracy of antibody tests may also be affected by what type of antibody test is used.
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). Currently, there are no COVID-19 antibody testing kits certified for home use in the UK. 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 infection. More research is needed to assess the 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. Home sampling antibody tests have recently been made available to people in some parts of the UK through the NHS after a positive PCR test result.
What about testing for international travel?
If you are planning to travel overseas, it is important to understand what COVID-19 testing you are required to do. This will vary depending on the most up-to-date requirements of both the country you are travelling to and the UK Government, or devolved administrations, on your return. These requirements can change at short notice, so you should make sure to check the latest guidance before you travel abroad.
What does this all mean for me?
Rapid lateral flow tests should be used regularly (for example, twice weekly) and before meeting up with other people to detect infection with SARS-CoV-2 even when no symptoms are present. If you have symptoms of COVID-19 (regardless of whether you are vaccinated), a PCR test is used to determine if you are currently infected with SARS-CoV-2 with greater sensitivity than a lateral flow test. This is requested from the NHS (or a private provider). Antibody testing can detect if you have previously been exposed to SARS-CoV-2 but may not be able to distinguish whether that exposure was from infection or vaccination. 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 and understand the testing requirements before certain activities, such as travelling.
Find out more
Find latest NHS guidance on testing for COVID-19 here.
Find latest information from the UK Government on getting tested for COVID-19.
Download and share our infographics
We've updated our 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.
Take a look and download our testing infographics
Public Engagement Manager, BSI