Which COVID rapid tests are best? – Expert Reaction

A Kiwi research team has weighed up rapid testing choices. Rapid antigen tests may miss up to 44% of cases – but this can be partly offset with more frequent testing, and appropriate use.

Standard PCR tests take hours in the lab to confirm a result. The team revealed that a rapid-PCR test – delivering results in 20 minutes – was just as sensitive as the gold-standard nose swab currently used in New Zealand, however the researchers say this test isn’t useful for testing large numbers at once.

The SMC asked the study’s authors to comment on their research. 

Dr Gary McAuliffe, Auckland-based clinical microbiologist and senior author on this paper, comments:

Note: Dr McAuliffe is a co-author of this paper. 

“The purpose of the paper was to look at how well these types of tests would pick up people infected with COVID-19, and to look at current and future ‘use-cases’ for them. Clearly, the situation has changed in New Zealand since we wrote the paper, but the discussion looks at use-cases in the scenarios we are now encountering: resource-constrained with higher prevalence of COVID-19 in Auckland and the Waikato, and lower rates elsewhere.

“The data we have gathered here is useful to help inform choices for hospitals looking at expanding their rapid-PCR capacity and for rapid antigen testing. It provides some estimates of how they would perform in picking up infected individuals, as well as some use scenarios that are currently underway or under consideration. This information is relevant to our current situation as there has been a fast move towards rapid antigen testing in scenarios such as those we explore in our paper.”

What did you learn about rapid-PCR tests? What are the main advantages and drawbacks?

“We looked at three rapid-PCR tests which were new to NZ at the time, and found that performance was variable dependent on the product. The advantages were an answer in 20 minutes, they are very useful for quick decision-making, and they are very suitable for testing people in hospitals to help decide on patient management pathways. However, as a single test can only be processed at a time, these aren’t suitable for testing large numbers of people at once.”

What did you learn about rapid antigen tests? What are the main advantages and drawbacks?

“Sensitivity is significantly lower than for PCR methods, which means you will miss some infected people. We looked at this against prior (pre-Delta) outbreaks and found this could be up to 45% of cases. This is the main limitation – though this can be offset to some extent by choosing the right scenarios and groups of people to use it in, and by more frequent testing.

“There are definite advantages to rapid antigen tests in our current situation as resource constraints are less of an issue compared with PCR, particularly if an individual is able to do their own testing. Quality, training and recording of results are very important factors in their successful use and in understanding their impact; in particular,  it is really important to have a mechanism to assess the whether rapid antigen tests, which if used widely do come at significant financial cost, prove worthwhile in reducing transmission.”

Which of the studied tests have been approved for use in New Zealand? How could rapid testing be more widely used currently or in future?

“The ‘Liats’ and ‘IDNows’ are both in use in New Zealand hospitals now; these rapid PCR machines and others are critical for hospital management of patients and given supply constraints, labs are actively looking for more alternatives.

“The Carestart, Abbott and Roche rapid antigen tests are in use in various pilots. As detailed in our paper, these may be useful for one-off testing at mass gatherings, or serial testing in educational, social care, and hospital settings. Outside the public arena, businesses may wish to consider testing staff as part of other COVID-19 mitigation measures.”

Would you suggest any changes to New Zealand’s current testing policy?

“Our testing strategy is changing to meet changes in prevalence and public health strategy; however it is very important that any changes in strategy are clearly articulated to the public, and performed in a planned and cohesive manner. At the same time it is necessary and appropriate to do this urgently to reduce current pressures on laboratories, allowing them to focus on high-consequence testing to support the public health response and healthcare settings. It is clear that rapid antigen testing could replace a significant proportion of asymptomatic testing currently being performed by PCR in the community.”

No conflict of interest declared. Dr McAuliffe is a co-author of this paper. 

Associate Professor James Ussher, Department of Microbiology and Immunology, University of Otago, comments:

Note: Associate Professor Ussher is a co-author of this paper. 

“Rapid ‘nucleic acid amplification’ tests (which includes rapid-PCR) for the SARS-CoV-2 virus are critical for keeping our hospitals functioning. Unfortunately, there is an ongoing ​global shortage of these types of tests, which is unlikely to be resolved in the short term. Therefore, it is critical that we continue to assess new products as they become available and to compare their sensitivity.

“Rapid antigen tests are also likely to play an increasing role in testing as we learn to live with COVID-19. There is, however, marked variation in the performance of rapid antigen tests. Things have moved on since our paper, however it’s important to recognise the various advantages and limitations of rapid antigen testing.

“In particular, rapid antigen tests are less sensitive compared with nucleic acid amplification tests, such as PCR, and they are also less specific – which can lead to false-positive results, especially in a low-prevalence setting. These factors influence the choice of tests and the use scenarios in practice.”

Conflict of interest statement: Associate Professor Ussher is Science Director of the Government-funded Vaccine Alliance Aotearoa New Zealand – Ohu Kaupare Huaketo, a partnership between the University of Otago, the Malaghan Institute and Victoria University of Wellington. He is also on the Government’s COVID-19 Vaccine Science and Technical Advisory Group. Associate Professor Ussher is a co-author on this paper.

Dr Erasmus (Ras) Smit, Clinical Virologist, Environmental and Social Research (ESR), comments:

Note: Dr Smit is a co-author of this paper. 

“The study was performed at a stage when New Zealand was still pursuing an elimination strategy and many of its conclusions were written in this light, especially when comparing what proportion of positive tests would be missed by rapid antigen tests compared to PCR Ct values obtained in previous outbreaks.

“The study confirmed what many other studies have shown in the past – that rapid molecular tests are more sensitive than rapid antigen tests. With the delta variant, the average Ct value of positive PCR results are much lower than before which also means that the sensitivity of rapid antigens tests will be better than most overseas studies have shown.

“The advantages over that the rapid antigen tests have over the molecular tests are the fact that they are simple to perform, you don’t need expensive handheld or benchtop hardware, they can be mass manufactured and therefore there is no shortage as there is with the rapid PCR reagents. Lastly potentially they can also be used for self-testing as is the case in some countries.

“In order to use rapid antigen tests effectively and safely as an adjunct to New Zealand’s testing strategy one must know that they are good with detecting someone who is infectious at the time of testing, but not good at detecting someone who is going to become infectious in day or two or those who have been infectious for longer than 5 to 7 days.

“To overcome this problem, it is best to use them shortly before a potential transmission opportunity, for instance just before flying – and not 72 hours before flying – or if you want to detect infectious cases in the community, then use them when you are symptomatic. To reduce the pressure on laboratories so that they can concentrate their PCR testing on the patients with the highest probability of being infected and thus give a quick turnaround of results, rapid antigen tests can also effectively be used in screening programmes of asymptomatic individuals who frequently come into contact with COVID-19 patients such as health care workers and border workers. To compensate for their reduced sensitivity in asymptomatic screening one would have to increase the frequency of testing.

“It’s best the rollout of these tests happens in a stepwise fashion. There is no reason why other rapid antigen test than the five evaluated should not be used, but to make sure that New Zealand only uses the highest quality tests the MOH has produced a set of quality markers which the assays need to fulfil to be imported. We fully support this approach as there are many lateral flow devices (the antigen test mechanism used in rapid antigen tests) for which there is no independent validation data available.”

No conflict of interest declared.