PCR swab test

Group testing for COVID-19

COVID-19 Group Testing

The early problems with testing for COVID-19 have been well documented. The physical limits to the number of tests that can be run through a lab represent an upper limit to the number of tests that can be processed each day. That limit meant that tests were limited to frontline health workers until May, in the UK.

Although many people are pinning their hopes on a vaccine for the virus, there are no guarantees that a working vaccine will be will become available this year. There are currently 170 COVID vaccination trials underway and (at the time of writing) 47 have progressed to human trials. The difficulties with testing any candidate vaccine are legion and Phase 3 trials, even if rushed, will take months to complete.

Mass Population Testing

That means that rapid testing will remain the only way to identify and control the virus in the meantime, so the limits to the number of daily tests that can be processed are hugely important. One study published in The Lancet suggests “To prevent a second wave the UK needs large-scale, population-wide testing”.

The UK can currently process nearly 350,000 tests a day and about 186,000 tests a day are currently being delivered. It sounds like there’s a lot of spare capacity. However, one study from Harvard University suggests that the USA needs to deliver 5m tests a day in order to re-open safely. Based upon our population size, that would equate to about 1m tests a day in the UK. Once again, we may face a significant shortage of testing capacity.

Pool Sampling

Group testing may help get around the capacity limits. Originally developed in the 1940’s by an American economist, Robert Dorfman, it applies mathematic principles to group tests together. Testing a group, rather than an individual sample, massively increases the number of tests that can be run.

In principle, the idea is very straight forward: pool samples together and test the pool. If the test is negative, nobody is infected and only a single test space in the lab machine has been used. If the test is positive, everyone in the group then needs testing individually. The process has already been authorised for use in the USA.

The Limits of Group Testing

However, principles don’t always apply to practice in healthcare and there are some notable difficulties with group testing. There may be a loss of sensitivity (accuracy) if too many samples are grouped together. Samples are difficult to label automatically when they’re grouped together, which can delay the testing process.

Group testing also only works well when the rate of infection is low. As the rate of positive results climbs, more individual tests are needed and the efficiency of the process reduces. That means that it is best suited to testing asymptomatic groups, where the risk of the infection is much lower than in symptomatic patients.

Mass testing of people who do not have any symptoms is currently the only way to identify, and stop, COVID-19 spreading amongst populations. Although the UK does not have any testing programme planned for people without symptoms, group testing may offer one mechanism under which it could be delivered.

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