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
