Author: Dr Mark Ali MBBS BSc FRCS CTh, Medical Director
Tests for covid-19 fall into two categories: antigen and antibody tests. For each test category, there are different methods that can be used.
Antigen tests detect viral RNA. Usually, this requires taking a swab from the nose and throat and then analysing the specimen in the lab using the polymerase chain reaction (PCR). You may see these tests referred to as PCR tests. They aim to answer: do I carry coronavirus now?
Antibody tests look for the immune response to covid-19. If a person is exposed to covid-19, their immune system will react in order to start clearing the virus. Initially, the innate (non-specialised, early immune response) system kicks in to start fighting the virus. The adaptive (more specialised) system is quickly behind, producing specialised antibodies that specifically target covid-19. Importantly, the immune system usually retains a memory of these antibodies. In this way, when the virus is met again, the immune system rapidly clears the virus before it reaches sufficient levels to cause symptoms or transmit to others.
An antibody test detects the presence of antibodies. If present, this shows the person has generated an immune response and is likely protected from reinfection and transmission.
IgM, IgA, IgG antibodies
You may well have seen the terms “IgM, IgA and IgG” when referring to antibody tests. We are quickly having to become armchair immunologists! “Ig” refers to immunoglobulin and “M”, “A” and “G” are different classes of antibodies (there are five in total). The classes function in different types of immune response or during different stages of the body’s defence.
IgM is found mainly in blood and lymph fluid and is the first antibody the body makes when it fights a new infection. IgG is the most common antibody. It is found in blood and other body fluids and protects against bacterial and viral infections. IgG can take time to form after an infection or immunization. IgA is found in the linings of the respiratory tract and digestive system, as well as in saliva (spit), tears, and breast milk.
Antibody tests for covid-19 will look for one or more of these classes of antibodies in order to detect an immune response. For covid-19, we are gaining more data about the time course and magnitude of the immune response in relation to infection. Most studies show that the vast majority of people produce a measurable response by day 20.
What does “test accuracy” actually mean?
Test accuracy refers to two separate concepts: sensitivity and specificity.
Sensitivity concerns the false negative rate and is expressed as a percentage: out of 100 truely positive patients, how many are detected as positive by the test? The higher the sensitivity, the fewer the numbers of false negatives. In the USA, sensitivity is often called positive percent agreement (PPA).
Specificity concerns the false positive rate and is expressed as a percentage: out of 100 truely negative patients, how many are detected as negative by the test? The higher the specificity, the fewer the numbers of false positives. An example here would be if the test detects an antibody response to a different but related coronavirus. The individual would be informed that they have antibodies to covid-19 when in fact it would reflect infection with a different but similar coronavirus. In the USA, specificity is often called negative percent agreement (NPA).
We also need to interpret the test results in context. The likelihood of a true positive or true negative will vary depending on the prevalence-how common the condition is within a population. For covid-19, we can estimate but these are not firm yet. This is explained in a great article here.
Why take an antigen test?
An antigen test detects the presence of viral RNA. It aims to see if the virus is detectable now.
The test is taken to:
- Diagnose infection
- Test following infection to see if the virus has been cleared
- Screen asymptomatic individuals to detect if they carry virus and are potentially infectious
Antigen tests are a crucial part of mitigating transmission as we come out of lockdown. We need the ability to detect the carriage of covid-19 and respond to it rapidly. As people return to work and society tentatively re-opens, robust testing programmes as part of a return to work strategy will be key to reduce spread and improve confidence amongst employees and customers.
When should I take the antigen test?
The highest, peak viral load is around the time of symptom onset. If using the test to diagnose, it is sensible to do an antigen test as soon as possible following symptom onset. Ideally the test should be performed within days 1-5 from onset of symptoms. Virus may be detected after this (and in some cases can be detected for a long time after) but the chances of detection decrease as the body clears the virus or it moves further down the respiratory tract (which requires a different sampling method).
What does a positive antigen test result mean?
A positive antigen test means that the covid-19 virus has been detected on the swab. These tests are highly specific- the specificity is 100%. This means that the false positive rate is negligible. If the result is positive, it is highly likely to be true.
The test detects viral RNA and does not tell the difference between active viral infection and “dead” viral remnants. It therefore cannot distinguish between someone who is infectious (active virus) and someone who is not (dead virus shedding). To do this, you would need to extract the virus particles and try to infect cells to see if this is possible.
What does a negative antigen test result mean?
A negative result could mean one of several different things:
- A genuine negative test result: the person does not carry covid-19. They may never have had it or they have had it and cleared the virus to levels low enough that it is no longer detected.
- A false negative result: sadly, the sensitivity of the test is around 70%. This means that out of every 100 people who take the swab, 30 are incorrectly identified as negative when in fact they carry the virus. The technique used for the swab is important. Nasopharyngeal swabs should be uncomfortable to take in order to get a good sample!
Test results should always be evaluated within the clinical context. If there is a high index of clinical suspicion, the test should be repeated.
- The test is sampling the wrong part of the respiratory tract. The distribution of the virus across the respiratory tract varies between patients and over the course of the infection. Even if a person is infected, the virus may only be detectable in sputum or nasopharyngeal swab but not necessarily both locations at the same time.
Why should I take an antibody test?
An antibody test can tell you if you have developed antibodies to covid-19. On an individual level, this is important and useful to know as most people who are infected with covid-19 will develop an antibody response that will provide some level of protection. Many people did not have access to swabs to diagnose covid-19 at the time of symptoms. This test may confirm what they have suspected. Alternatively, perhaps the person has had contact with someone but never developed symptoms. It is also likely many people have been asymptomatically infected.
There are some cases where PCR antigen tests are negative but antibody tests are positive. Therefore antibody tests may also be used to diagnose infection now but they should only be used in conjunction with clinical information to rule in infection rather than rule out.
When should I take an antibody test?
We are constantly learning more about the immune response to covid-19: the timecourse, magnitidue, variation with age and disease severity. Most studies suggest you should wait at least 14 days following symptom onset and ideally four weeks as some people do take this long to produce a detectable antibody response.
We do not know how long the antibodies last but it is reasonable to suggest (based on our knowledge of immunology and previous similar viruses) that they will last a few months to a couple of years, if not longer.
What does a positive antibody test result mean?
A positive antibody result likely means that an individual has generated an antibody response to covid-19. The specificity (false positive rate) of the tests is improving and our manufacturers give a specificity of 100%. It is early days and there is the possibility that a test detects antibodies to closely related coronaviruses instead.
Do the antibodies provide immunity to covid-19? We hope so but we do not know for certain if the antibodies are protective. Based on our knowledge of immunology and similar coronaviruses, it is highly likely that the antibodies will afford protection. The World Health Organisation (WHO) has stated, “We expect that most people who are infected with covid-19 will develop an antibody response that will provide some level of protection.”
We would still emphasise that until we know more, individuals should continue to follow government guidelines and act as if they could catch covid-19 and/or pass it on to someone else.
What does a negative antibody test result mean?
A negative test result means that the antibodies to the virus that causes covid-19 were not found in your sample.This may be for the following reasons:
- You have not been infected by covid-19
- A timing issue: you have been infected by covid-19 but it is too early to detect the antibodies. The test should be viewed in clinical context. It may be worth repeating the test in a couple of weeks if there is a high index of suspicion of disease.
- You have been infected by covid-19 and cleared the virus without producing antibodies (using the innate immune system). The immune system is extremely complex and there are lots of ways it can generate immunity, antibodies being just one.
- Some health conditions may make it more difficult for some people to produce antibodies to the disease
- The test result is a false negative. The sensitivity of the tests is improving and the manufacturers of our test give a sensitivity of 97.5% at 14 days.
The antibody response to covid-19 has been studied in those with confirmed disease. As efforts were focused on testing those who were hospitalised, these patients represent the more severe end of the spectrum with likely higher antibody levels. As testing becomes more widespread, the antibody tests can be developed to ensure they function well for all of the population.
What is in the pipeline? What will future tests look like?
An ideal test will be highly sensitive and specific (low numbers of false negatives and false positives respectively). The test will be a point-of-care (POC) test that can be self-administered and results available within 15 minutes. Ideally, the test will be non-invasive and the results not too dependent on correct technique.
The good news is that this is a challenge the entire global scientific community is working on. Antigen tests that can be POC will not face the issues of requiring transport, expensive laboratory equipment, skilled operators and reagents that have run out. A rapid test to see if someone is carrying covid-19 will be a game-changer: individuals can know there and then if there is presence of covid-19, which will provide reassurance or enable immediate self-isolation. This is how we will truly get on top of transmission and drive down the “R” (reproductive number). For employers trying to get employees back to work, reassuring them and customers, these tests will truly supercharge a return to work strategy.
There may be a shift in how tests are performed. For example, companies are working on salivary tests to detect antigens, which will be less operator-dependent and non-invasive.
Antibody tests are being rapidly improved as more and more samples become available. Again, POC tests will be the ideal, potentially using saliva rather than invasive fingerpricks.
We are continuing to watch this ever-changing field in order to offer the best tests available at the time.