The use of Rapid Antigen Tests in NZ

Updated: Oct 9

This is the second in a series of guest blogs by pathologist, Dr Arthur Morris

There is a lot of attention being given to rapid antigen testing (RATs) for COVID-19 in Australia and New Zealand currently, particularly as it is being widely used in countries such as the UK. Rako Science's view is that different tests have their roles in different circumstances.

The key questions for any test are:

  • What is the purpose of the testing, eg. is it for a screening or for diagnosis?

  • What is the setting for the testing, eg. is it a workplace or home?

  • What is the prevalence of the disease in the tested population, i.e. how common is it?

If you are looking to secure a venue or protect a workspace workforce and ensure it is clear of COVID-19, RATs are unlikely to meet this goal. The science shows that RATs only reliably pick-up people who are highly infectious. A “negative” test might not be accurate and may give the person false confidence while they go on to infect others.

With a low prevalence of COVID-19 in the community if the aim is to protect a workplace then a highly sensitive test is needed to detect infections early and break chains of transmission. Early detection allows public health authorities to isolate people before they become infectious. That is the empirical experience of the University of Illinois programme here which has completed 2.3 million tests on a 46,000 campus population, keeping the University open and preventing campus transmission.

RATs are ideally suited for populations with high prevalence of infection and when there are minimal public health controls. In this situation, governments want businesses and families to take personal responsibility by purchasing RATs for the workplace or the home.

In this figure, from a paper based on daily sampling of early SARS-CoV-2 infection here is a comparison of the saliva protocol used by Rako Science and antigen testing. You can see that when infected people are less infectious (negative viral culture) antigen tests identify only 28% of them. The performance is much better, 93% detection, when they are highly infectious (positive viral culture). Early detection prevents transmission.

In another study here testing was performed daily on vaccinated and newly vaccinated people experiencing a "breakthrough" SARS-CoV-2 infection. The RAT was compared to the PCR assay used by Rako Science. This study found RAT identified 15% of newly vaccinated positive cases and 12% of vaccinated positive cases while less infectious (negative viral culture). For highly infectious cases (positive viral culture) RAT identified between 85% of newly vaccinated positive cases and 78% of vaccinated positive cases.

In a recent systematic review and meta-analysis here, the pooled comparative sensitivity of RATs vs. PCR tests, such as the Rako assay, was 76% when the manufacturer’s instructions were followed. As with the studies referenced above, RAT sensitivity is higher with samples with a high virus load, 97%, to only 51% and 21% with medium and low levels of virus.

Implications for use

These results have significant implications for communities and workplaces with low prevalence COVID-19 and where early detection is the purpose of testing, eg. workplaces where a positive case can be hugely disruptive such as hospitals, energy generation plants and aged care and food production facilities.

In other countries, RATs have been used in symptomatic screening as a point of care test when a patient is displaying symptoms and the purpose is to enable rapid triage of the most infected and infectious patients. The characteristics of RATs are desirable in these circumstances. The fast result allows the health care professional to manage the infection control risks rapidly. In New Zealand anyone with symptoms should have the mandated nasopharyngeal test and be placed into appropriate infection control precautions while the RAT result is awaited.

Rako Science’s view is that all tests have a place including RATs. However, RATs are not suitable as surveillance tests in low prevalence groups where your testing aim is maximum sensitivity. That is when you need the most sensitive test to identify infections and block chains of transmission.

Currently it is illegal to import, manufacture, supply, sell, pack, or use a point of care test, such as RATs, without approval of the Director-General of Health. RATs are currently being trialed and evaluated in New Zealand, and we strongly recommend that their use has a clear purpose and clear understanding of the setting where they are used. We look forward to the results of these evaluations and how such tests can be added to testing options to tackle the evolving epidemiology of COVID-19 in New Zealand.

Dr Arthur Morris

Arthur Morris is a pathologist, clinical microbiologist, with a long-term interest in infection prevention and control. He is the supervising pathologist for IGENZ.

In January 2021, IGENZ Ltd, a contracted testing laboratory, achieved IANZ accreditation for saliva testing using the Rako Science methodology, based on validation work that used a set of contemporaneously collected paired saliva and nasopharyngeal samples.

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