Rapid antigen testing in COVID-19 population risk management

by InScience | Oct 12, 2021 | News

As the sole screening mechanism for COVID in real-time population management, PCR testing is not enough. Rapid antigen testing plays an integral, complementary role to PCR confirmatory testing. Regular testing with rapid results, multiple times a week, can greatly help prevent and control COVID outbreaks and avoid costly lockdowns and quarantines.

PCR Vs Antigen

PCR tests are  considered the “gold standard” of testing as they are extremely sensitive and good at detecting the presence of the virus at extremely low levels of viral load.  Because these tests can detect viral fragments even without virus, individuals will likely test PCR positive long after they have transmissible virus. According to Harvard School of Public Health Professor Michael Mina, MD, PhD, 40-80% of the time a patient is PCR positive, the individual is post infectious. Though sensitive and good at detecting if a person was infected by the virus, PCR tests are expensive and slow, making them less than ideal when screening large populations.

Rapid antigen tests, on the other hand, are excellent at detecting infectiousness and decreasing community spread. Unlike PCR tests, rapid antigen tests are more affordable and provide results in 10-15 minutes, allowing for the ability for regular, widespread testing. In a study by the University of Colorado BioFrontiers Institute and Harvard Chan School of Public Health, when used twice a week, widespread rapid testing reduced the degree of infectiousness of the virus by 80%, compared to twice (2x) weekly PCR only testing (with 48 hour results), which reduced infectiousness by 58%.

For population management, serial rapid testing is more effective than infrequent, PCR only testing.

Decreasing the degree of COVID population spread

Due to the frequency and timeliness of results, rapid antigen tests add value in triaging and contact tracing infectious individuals. These tests serve as a crucial screening tool and a key part to stopping COVID-19 from spreading.  Frequent rapid antigen testing can reduce the degree of infectiousness of the virus considerably more than a PCR-only testing protocol.

Accessing rapid Covid tests for your workplace

Incorporating frequent rapid antigen tests into workplaces as a  testing protocol can help slow the spread of COVID-19 in our population and provide a much needed risk management tool for workplaces providing quick and accurate results that provide a measure of contagiousness and reduce the rate of infection.

Rapid antigen testing? The big picture

by InScience | Oct 10, 2021 | News

• Testing for(target) is different than PCR targets to detect virus
• Antigen testing helps you proactively protect others
• And a form of others working to protecting you — them figuring out if they can transmit virus to you- Surveillance
• Rapid and economical because no lab equipment is necessary —  at your workplace
• Ideal for more regular testing that also increases sensitivity  

First, let’s define a key term: an antigen. It’s the part of an infectious agent that your body recognizes to trigger an immune response, so this can be something like a protein on the outside of a virus.

What a rapid antigen test does is identify antigens in a human sample by flowing that sample over a set of molecules that serve as probes because they can bind to the antigen of interest–if it’s in the sample.

Many rapid antigen tests are up to 95% specific

This usually means that the infectious agent has to be exactly that–infectious, because it needs to be viable enough for whole antigen to be present. This is in contrast to PCR which is detecting the genetic material of an infectious agent like a virus in a sample, because the RNA of a virus (like SARS-CoV-2!) can hang around in a sample for longer than it actually is able to infect a host, simply because of how RNA decays in contrast to something like a protein that would be the antigen on the outside of such a virus.

As for the rapid piece, these tests are relatively quick (usually under 30 minutes) because all the molecular pieces needed for analysis are in the test device itself, and there’s no need for sample processing or temperature control or changes to drive a chemical reaction, for example. By design, if there is antigen in the sample to bind to the “probe” molecules, a simple internal chemical reaction that is already set up and ready to go then displays colors in the form of easy-to-read lines for a quick result!

What can Rapid Antigen testing do for New Zealand’s efforts to eliminate or live with Covid 19

by InScience | Oct 4, 2021 | News

WHAT IT IS

Rapid antigen tests (RAT) detect the presence of certain proteins of the Covid-19 virus in symptomatic patients using an immunoassay test principle. We have been using this technology for decades for rapid and accessible screening for HIV, drugs, pregnancy to name but a few applications

The tests can be performed by people trained to use it and/or health professionals outside of a lab and produce a result within 15 to 30 minutes.

The RAT can detect the virus in the acute phase of infection, especially in the week before symptoms are apparent, and the first week of symptoms being apparent so an ideal tool for surveillance in our NZ workplaces. NZ workplaces and our health professionals are familiar with performing screens and understanding what they mean. They are crying out for a means of risk management and ability to comply with surveillance testing mandates or their own workplace risk minimisation efforts with economy of $ and time. 

Rapid COVID-19 antigen testing has been in use for some time now in the UK, US, Canada, and other countries more recently Australia where it has increased detection rates. RAT has been validated by TGA, FDA and other recognised and credible validating organisations before the test devices were applied in other countries as fit for purpose.

The Therapeutic Goods Administration in Australia has approved a substantial number of rapid antigen tests and NZ has many of these submitted for approval on the same basis. The NZ government however fails to approve RAT for use in NZ under any circumstances

HOW IT IS DIFFERENT FROM PCR

Polymerase chain reaction tests, known as PCR tests, are considered the gold standard for diagnosing Covid-19.

These tests have a higher sensitivity (detect at low viral load) and can detect even old remnants of the virus after active infection.

PCR tests detect the presence of the genetic material of the actual virus and are good at finding the virus early. They can sometimes detect the virus in someone before they fall ill or are symptomatic.

However, unlike RAT, which provides a rapid result, PCR tests can take hours, or sometimes days, to get results.

As well as providing rapid results, antigen tests are also cheaper and designed to be used at point of care or onsite.

Naso pharyngeal testing is an unpopular test sample for regular interval/surveillance testing Photo: Tim Hunter.

WHAT RESEARCH SHOWS ABOUT RAT ACCURACY

Although Rapid Antigen Tests provide a rapid result and can use either a nasopharyngeal, saliva or nasal sample, there is a risk of false positive and false negative results. False positives and false negatives are a factor with the gold standard PCR also but the checks and balances (controls and duplicates) in PCR testing are routinely applied to reduce this. PCR is the gold standard and as such essential to confirm any “not negative/unconfirmed” or screen positive RAT results. PCR false positives can be related to detection of fragments from past infection and false negatives may result from inadequate sampling.

Therefore, rapid antigen tests cannot be used or relied upon as in the same way or as the only way to definitively diagnose a COVID-19 infection and any not negative/unconfirmed rapid antigen test result must be confirmed by the gold standard PCR test. Rapid Antigen test can however be a very efficient system, effective and widely employed strategy to economically, time and cost wise, survey a geographically and socially diverse and distanced population as we do already use a screen/confirm process for major health screens i.e., cancer, workplace drug testing, Police roadside alcohol testing

Unfortunately, the one swab cannot be used in both the RAT and PCR tests however there may be potential to use one (the same) saliva specimen when collected.

It is not acceptable practice to rely upon a repeat of the rapid antigen test to confirm an initial not negative/unconfirmed rapid antigen test (RAT) result and it must always be considered just that, a not negative/unconfirmed result. Any such RAT results need follow up with a PCR test and preferably at a community testing centre.

POTENTIAL RAT USES

Over the past few months there has been a rapid expansion internationally of rapid antigen testing for use in the health and non-health sectors to minimise risks of outbreaks, provide accessible, less stressful, and more affordable testing for the man on the street but especially for business surveillance.

There are no plans communicated by NZ Government, to allow businesses wanting this option to use rapid antigen testing. Enabling workplace- based and pharmacy-based testing or in time, home or self-testing should be given more oxygen.

RAT testing is proving a remarkably effective layer in surveillance of Covid 19 prevalence in the essential services community especially for asymptomatic people in other developed countries. When conducted for workplaces or those at risk of exposure in those workplaces as regular interval tests, RAT is extremely specific and sensitive. Regular interval testing every 3-7 days with Rapid antigen tests raises its ability to detect active infections in asymptomatic people (sensitivity) i.e., before they get any symptoms. It is close to the sensitivity longer interval, PCR testing affords us. This (RAT) could if approved for use in NZ as it is in other developed and >70% plus vaccinated countries, in places where lots of people pass through, for example, airports, schools, industry settings such as transport and supermarkets and for events be an invaluable part of living with Covid. This type of testing and regular affordable testing is an especially important layer and key to casting the surveillance net wider.

Rapid antigen testing is already used in countries with high vaccination rates for travel surveillance

ADVANTAGES AND DISADVANTAGES OF WIDESPREAD USE OF RAT

Rapid antigen tests are quick and easy to perform, with turnaround times of 15 to 30 minutes and with regular interval testing performs well for surveillance of a wider number of people. The manual performance of a RAT and its requirement for visual interpretation make it both more affordable but subject to human error.

Specimens that can be used for Covid tests– 

Saliva specimen-based RAT or PCR is popular and requires at least 30 minutes without prior eating or drinking before specimen collection. Nasopharyngeal swabs are not as well tolerated by the person being tested and sampling conducted by a Collector. 

There are RAT that use a specimen from just inside the nose, so not invasive is more friendly than the more invasive NP collect or the wait sometimes required if the person has had food or drink within the previous 30 minutes The Nasal RAT is capable of use without loss of accuracy or sensitivity and is proving extremely popular. 

At this stage of the pandemic, health messaging by Government agencies is encouraging people with possible Covid-19 symptoms to have a PCR test as the gold standard with the best accuracy, sensitivity and specificity. This why PCR is indicated for confirmation of RAT. PCR routinely runs tests in duplicate and/or confirms initial PCR positives. PCR can be performed onsite using an open and portable platform. 

Last word

Once vaccination coverage in New Zealand reaches higher levels, and restrictions are lifted, RAT will be useful in those situations where groups work and play, and crowds pass through. The United Kingdom recently changed their travel covid testing protocol after months of a combined RAT /PCR protocol i.e., RAT immediately before returning to UK from European destinations and PCR two days after their return to one where the traveller can conduct a RAT test pre return travel and then again after return at an affordable cost and greater uptake. 

We know the Rapid antigen tests are less sensitive than the gold standard PCR test, meaning it is possible some cases of Covid will be missed initially but regular interval testing as demonstrated by the UK travel protocol increases that sensitivity to particularly good positive and negative predictive values.