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02-10-2020
Understanding the PCR Antigen (virus) Test
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Covid Testing Network have been supporting workplace testing since the start of the pandemic and we’ve put this guide together for employers and occupational health providers who are in the early stages of deciding / helping clients to decide whether they want to offer workplace testing* to their staff. It offers a basic introduction to key terms surrounding PCR antigen testing in a workplace context and some broader FAQs from our recent conversations around test planning. It is designed to help you understand a little more of the landscape before you work out whether testing is for you.

 

 *There are two approved tests used in the detection of COVID-19. Both are introduced below but this guide is focussed on the PCR antigen test because, as the only test currently relevant for outbreak prevention, it is what is being used in the majority of workplace testing scenarios. 

 

Antibody vs PCR Antigen testing 

As indicated in the Government guidance on workplace testing, there are currently two tests that have been approved for coronavirus detection: the antibody test and the PCR antigen test (also referred to as the virus test). Both require lab processing and should not be confused with home-use "self-test" kits which are not sent away for lab processing. As the guidance states: “none of these COVID-19 kits have currently received a CE mark and so there are no self-test kits in the UK suitable for home use."

 

What is the Antibody test? 

It is a lab-processed blood test that indicates whether someone is likely to have already HAD COVID-19.

 

How does the Antibody test work?

"Antibodies are proteins produced by the immune system in response to an infection" e.g. a virus. Someone with SARS-CoV-2 (the virus that causes the disease COVID-19) will produce antibodies that respond specifically to this virus, a small number of which usually remain post-recovery, acting as a blueprint in case the body needs to make them again. The antibody test detects the presence of these virus-specific antibodies in the blood. 

 

Are there any issues with the Antibody test?

The current evidence is inconclusive as to whether a positive test result means you will be immune to future infection. The advice on social distancing is exactly the same for positive and negative test results. 

Antibody tests are currently used as a way of understanding the virus and how it spreads. They are not currently recommended for outbreak prevention or control.

 

What is a PCR Antigen test?

It is a lab-processed swab test designed to detect SARS-CoV-2. Where the antibody test looks for proteins that indicate a past infection, the PCR antigen test looks for traces of the actual SARS-CoV-2 virus the ‘antigen’ and is designed to confirm whether or not someone HAS the virus in their system at the point of testing.  

 

How does the PCR Antigen test work?

A PCR (polymerase chain reaction) antigen test is designed to detect the DNA belonging to a specific antigen.

Samples are collected " usually by swabbing both the throat and nose" and there are then two stages to lab processing. In stage 1, trace genetic material is isolated and extracted from the sample using chemicals which convert viral RNA (if present) to a DNA copy. In stage 2, this material is then amplified many thousands of times, increasing the number of any DNA copies that exist. This makes it possible to detect the virus even at an early stage of infection when the viral load (the amount of virus in the sample) may be low.

 

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Are there any issues with the PCR antigen test?

Performed correctly, the PCR antigen test is extremely precise: current research suggests over 99% accuracy with a false positive rate close to 0. In short, if there is virus present in a sample, it is almost guaranteed to be detected and if there is not, it won't.

 

False positives (a positive test result for someone who does not, in fact have the virus) do occur and when they do, they are usually thought to stem from poor lab handling.

 

False negative rates are higher than they are for false positives: research is ongoing but the reported range currently stands between 2% and 33%.  It is rare for a reputable laboratory to report high false negatives rate as most false negatives are thought to be the result of poor sample collection processes in the workplace, but they can also result from poor lab handling.  Covid Testing Network have built a network of high quality laboratories that are not only UKAS ISO accredited, but also validated by NHS England specifically to conduct testing for Covid-19. 

 

What is the difference between Symptomatic and Asymptomatic testing and why do most workforce testing programmes focus on routine asymptomatic testing?

If you are currently displaying NHS listed symptoms of COVID-19 you are considered to be symptomatic. You will already be self-isolating and - depending on test availability - you / your employer may be able to access a PCR antigen test through NHS Test & Trace to ascertain whether or not you have the virus. At the moment, you can only get a test through NHS Test & Trace if you are symptomatic. If you do not have any of the listed symptoms of COVID-19, you are considered to be asymptomatic and you cannot access a test.

 

Asymptomatic people either: a) don’t have the virus (likely test outcome = negative) ; b) are pre-symptomatic: they have the virus but it’s early stage and symptoms haven’t developed yet (likely test outcome = positive) ; c) have the virus but will never develop symptoms (likely test outcome = positive)

 

Most workplace testing is aimed at identifying people in categories b and c via routine (weekly) PCR antigen testing. Routine testing gives employers the ability to monitor their workforce: should a positive result be returned; they can act swiftly to prevent a workplace outbreak from happening. Routine testing also offers continued reassurance to employees, increasing their confidence in the safety of their work environment.  

Asymptomatic individuals were able to access tests earlier in the year - the criteria change in relation to virus spread and test capacity - however regular asymptomatic testing for non-healthcare workplace settings has not thus far been available through Test & Trace and is unlikely to become so.


What is PCR antigen pool testing?

Instead of processing each sample individually "as described above" material from a batch of samples is combined in stage one of the lab process and the PCR test is performed on this single combined sample. Only if this test returns a positive result are the batch samples tested individually to ascertain the source(s). For a large workforce, cost savings can be significant as this method allows the testing of more people using less resources and less lab time.

 

What is community spread and why is it relevant?

If someone catches the virus but hasn’t knowingly had contact with someone who is sick, the virus is deemed to be spreading in the community. If spread is high and cases spike, local lockdowns may be enforced. In these circumstances, some workplaces may consider set up emergency testing programmes until the local authority has got the situation under control.

 

If a lot of people have had the virus in this area, will we have herd immunity?

The short answer is no. Herd immunity occurs when a large percentage of a population becomes immune to a disease either as a result of natural infection or vaccination. This percentage threshold varies by disease but is generally extremely high, at approximately 95% for measles, 80-85% for polio, which is a less contagious disease.

With Covid-19 being a new virus introduced into the UK, everyone is deemed as being susceptible to it.  Herd immunity will only therefore be deemed to be relevant once the vast majority of the UK population has been proven to have had it and survived, so that sufficient levels of antibodies are created in their bodies.

In the absence of a vaccine and bearing in mind the lack of conclusive evidence that people who return a positive antibody test are immune (or that conferred immunity will last), the UK is a long way from the possibility of herd immunity.

 

Are PCR tests really that accurate? 

As discussed, the likelihood of the PCR antigen test returning a positive result for someone who does not have the virus is close to 0. Similarly, if there is any trace of virus present on the swab sample taken from an employee, the test should detect it. The tiny number of false positives are generally thought to result from a lab error such as cross contamination. The take-away here is that the quality of the lab you select really matters.

 

False negative results are more likely than false positives partly because there are more factors in play to cause them. False negatives can result from a lab processing error but if you have chosen a high-quality lab, more often they will stem from poor sample collection and / or handling at the workplace end of the process: employees needs to be shown the correct method by which to self-swab; samples must be carefully sealed, stored and transported properly; accurate records must be sent to the lab. Therefore we strongly recommend that all employers thinking about workforce testing engage a reputable occupational health provider to manage lab selection and operational delivery and ensure programme efficacy.  

 

At Covid Testing Network we assist end-to-end with workplace testing plan design and delivery management. Our network of labs offer high quality processing services and in addition to meeting all of the Government’s recommended accreditations, have also all passed the Department of Health and Social Care’s rigorous approval process. If you would like to discuss workplace testing with us, please reach out to our Head of Client Management, Ami Jayakrishnan

 

Further reading

Antibody testing (British Medical Journal)

Sample pooling for testing SARS-CoV-2 in asymptomatic people (The Lancet)
Investigation of SARS-CoV-2 care home outbreaks (The Lancet)

Government guidance on workplace testing

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