Molecular PCR, Antigen and Antibody COVID testing explained – which test should you choose?

There’s so much noise out there about COVID testing that it can be really challenging to understand the science. That’s why we’ve asked Dr Greg Fitzgibbon, Clinical Director at MyHealthChecked to talk to us about how coronavirus testing actually works, and what’s the difference between molecular (PCR), antigen and antibody covid tests. Read on to learn the hows, the whats, the pros and the cons, so you can decide what’s right for you…

Let’s start with the basics: how do they test for COVID?

There are two broad categories of COVID tests: the diagnostic tests and the antibody COVID tests. And that’s not all; within the diagnostic category there are two different kinds: molecular (PCR) and antigen COVID tests.

So let’s take a closer look at these different tests, the methods they use, and what that means in practice. Then you’ll have the information to decide what’s right for you – because at the end of the day the best COVID test for you is the one that best meets your needs and circumstances.

Diagnostic tests show if you have active COVID

Diagnostic tests look for active viruses – they tell you (diagnose) if you are infected with COVID-19 at the time of testing

Molecular PCR or Real-Time PCR COVID tests – the accurate diagnostic.

Real-Time PCR or molecular PCR tests look for the presence of COVID-19 virus’s genes at a molecular level. They do this using a process called qPCR which stands for Quantitative Polymerase Chain Reaction. PCR tests are the most accurate and sensitive tests available, though they take longer to provide a result.

This is the process we use at MyHealthChecked, because it’s 99.9% accurate, so there’s less risk of false negatives, thus a less chance of accidentally spreading the virus.

PCR testing usually starts by taking a swab that’s sent to a laboratory for processing and evaluation – which can take time. However, PCR tests are much more sensitive than other types of test, detecting right down to the most minute levels of virus presence, which is particularly useful in the earliest stages.

COVID antigen tests – the fast diagnostic

An antigen test is a way of detecting the virus by looking at its proteins rather than genes.

The antigen test works by looking for the proteins that are found on the surface of the COVID-19 virus in order to ascertain its presence. It is these proteins that are called ‘antigens’.

After the swab is taken from the nose and/or throat, it is mixed in with a solution known as an ‘extraction buffer’. Then, it is dropped onto a paper strip found in the testing device. As the sample flows down the paper strip, whatever is in the sample gets bound to one or two of these three lines. If the presence of COVID-19 is detected, then two lines are seen. If not, then you only see one line.

These tests can be quicker and more convenient. They can be done in minutes, on-site, using a similar method to home pregnancy testing. However, there is currently limited data on the reliability of antigen tests in clinical practice and they can have limited sensitivity compared to the ‘molecular’, PCR-based tests.

The downside of these tests – when it comes to accuracy and sensitivity – is that they can give either false positive or false negative results. False positive results aren’t ideal because they cause distress and inconvenience, but false negative results are the big worry. Imagine how many people you might pass it on to, and how many more people they might infect in turn. If you have vulnerable people in your bubble, accuracy has to be one of the most important things to look out for.

COVID antibody tests – identifying historic infection

Antibody testing looks for the signs of past infection – it cannot diagnose if you have ‘live’, active COVID-19.

When our bodies are under attack from a particular infection – for example, flu – we produce antibodies to fight that specific infection. These antibodies can remain in our system for long periods of time to keep us immune and prevent reinfection. An antibody test looks for these, to identify if we have had a disease in the past.

COVID antibody tests detect the presence of antibodies specific to this coronavirus in our blood. This shows that a person has had the disease, so may have immunity to the virus. However, because this virus is new to us the chance of reinfection is less well understood than with a more familiar virus like the flu.

So though a positive antibody test is interesting to your GP if they are looking into the causes of symptoms like fatigue or breathlessness – it offers no guarantee that you’re now ‘immune’ to COVID-19.

What’s the best COVID test for you?

Diagnostic and antibody tests are used in different ways in the medical world, and can both be very useful. But when it comes to personal COVID testing, it all comes down to what you want to learn, and why.

Best COVID tests to protect others

If you have vulnerable loved ones, you might want to take a PCR test before making a visit. In this case, you need to know if you have COVID at present – and you want to be as certain as possible about your result, so it’s best to choose the highest accuracy you can find – 99.9% in the case of our PCR tests.

In ‘high risk’ cases, if you want to be really sure where you stand, you’ll need to isolate for five days beforehand, and while you wait for results to come back before you visit your loved ones. Remember, all diagnostic tests can only show if you are infected at the time you take the swab – so for the greatest peace of mind, you need to give the virus time to incubate beforehand, and avoid infection afterwards.

Best test if you want to know if you have had the virus

So a month or two ago, you felt a bit off colour, and now you’re really tired all the time, and feel very short of breath. You’ve read a lot about asymptomatic COVID – and long COVID – and now you’re beginning to put two and two together.

This is the time when a COVID antibody test might be useful. It can tell you if you have had the disease in the past. However, if you are concerned about tiredness and breathlessness, the best first step is calling your GP, as there are many possible causes, and it’s best to get checked out by a healthcare professional. If they think it’s relevant, they might organise a COVID-19 antibody test on the NHS for you, too.

Best COVID test to know if you have the virus now

Sometimes, life means our COVID exposure risk increases – and that can be really worrying.

Perhaps you’ve had a big work meeting where you couldn’t maintain distance. Maybe there has been a positive case in your kid’s class at school. Or you have ended up in a high risk area, and haven’t been as great with the PPE as you should have been. In these cases, you’ll have to weigh up:

If you want the fastest possible answers, and can accept the risk of inaccuracy?

Then antigen tests might be suitable, especially if you have a large group to test and a limited budget.

Is getting a result you can really rely on your primary motivation?

In this case, look out for PCR tests – these are the tests with the highest levels of accuracy.

Best COVID test for flying

If you’re planning on flying, most airlines will ask for a PCR test to meet Fit to Fly regulations – you can read more in the article What is a PCR test certificate – and when do you need one?

Did you find this guide useful? Do you have any questions you would like us to answer? Please don’t hesitate to contact the MyHealthChecked team.

References https://www.gov.uk/guidance/coronavirus-covid-19-test-to-release-for-international-travel