Rapid COVID-19 tests could be as effective as PCR lab tests

Rapid COVID-19 tests could be as effective as more accurate PCR lab tests if enough people get tested, study finds

  • Rapid antigen tests can be just as good at bringing down COVID-19 infections as more-accurate PCR tests, a new study finds
  • Antigen tests – which identify proteins on the virus’ surface – are cheap and provide results in minutes
  • But a lot of people need to get tested daily – at least 0.5% of the population, according to modeling from researchers in India
  • This frequent, high-volume testing can overcome the rapid tests’ lower accuracy
  • The researchers suggest that low- and middle-income countries should use rapid tests instead of more-expensive PCR tests to control Covid

If rapid antigen tests are used in a high enough volume, they may be just as good at reducing COVID-19 infections as more-accurate PCR tests, a new analysis suggests.  

Researchers at Ashoka University and the National Centre for Biological Sciences TIFR in India found that if at least 0.5 percent of the population is tested daily with these rapid tests – and a couple of other conditions are met – infections can be significantly reduced.

Rapid tests may not be as highly accurate as PCR tests, but they are much cheaper to produce and provide results more quickly.

The findings suggest that low- and middle-income countries could concentrate on using these rapid tests in order to control Covid as the Indian ‘Delta’ variant spreads.

Rapid tests may be just as good at PCR tests at controlling COVID if enough people are testing, a new study finds. Pictured: A patient gets tested at a clinic in Los Angeles, California

Regular rapid testing combined with quarantines for those who test positive could bring down Covid infections in a population, the modeling study suggests

If a person needs to determine whether they are infected with the coronavirus, polymerase chain reaction (PCR) tests are considered the gold standard.

These tests work by identifying genetic material from the virus in a patient’s mucus or saliva. They’re highly accurate, able to catch almost 100 percent of cases.

PCR tests have some major drawbacks, however. They’re expensive to make and it takes time to send a patient’s sample out to a lab, meaning that patients may not get their results for days.

As a result, many public health experts have pushed for using rapid antigen tests in certain settings.

Antigen tests look for a protein on the surface of the coronavirus. They can cost as little as five dollars and provide patients with results in as few as 15 minutes.

While PCR tests are better for telling a patient in the hospital whether they have Covid, antigen tests may be better in schools, workplaces, and other settings where leaders want to constantly monitor for potential outbreaks.

These tests are less accurate than PCR tests, however. Antigen tests have a lower sensitivity, meaning that they’re likely to give a negative result when, in fact, a patient does have Covid.

Depending on the antigen test, this may happen for as many as one in three patients.

However, if enough people are tested – and they’re tested often – these rapid antigen tests can still identify enough cases to push down Covid numbers in a population, a new study suggests.

For the new study, published Thursday in PLOS Computational Biology, the researchers used mathematical modeling to examine the impacts of different testing scenarios with both PCR tests and rapid antigen tests.

The study was inspired by two states in India that relied on antigen tests – and had lower case numbers in 2020. 

This suggested that antigen tests might be better at bringing infection rates down, the researchers hypothesized.

As the models were based on India’s Covid trajectory, the researchers assumed that at least 20 percent of the population already had the disease – and now has some degree of immunity.

The models examined different testing frequencies, shares of the population participating in regular testing, mixes of PCR and antigen tests, and more.

If at least 0.5 percent of the population is tested daily, infections will go down, the models show. Non-pharmaceutical interventions (NPIs) provide additional benefits

The researchers found that antigen tests alone can indeed successfully bring down infection rates.

The antigen test-only scenario had similar outcomes to PCR testing, both reducing overall numbers of people who caught Covid and reducing peak infections – meaning that hospital systems would be less overwhelmed.

But a few conditions are needed for frequent antigen testing to be a success.

First, at least 0.5 percent of the population should be tested every day – and the number of people tested should go up if daily infections exceed 0.5 percent of the population.

Second, specific groups of people should be prioritized for testing. This includes anyone with Covid symptoms, those who recently traveled outside the country, contacts of confirmed cases, healthcare workers, and other frontline workers.

Third, some PCR tests should still be available. These tests should be reserved for patients in the hospital and others who urgently need an accurate Covid diagnosis.

And finally, anyone who tests positive should go into quarantine – along with the rest of their household.

Antigen tests must be used frequently and for many people to bring down infections. Pictured: A mobile Covid testing van in Queens, New York

These findings align with other modeling studies showing that, when a lot of people use antigen tests – and they do it often – cases can go down.

No country has yet tried antigen testing on such a big scale, but the strategy has been successful at some U.S. colleges and other small locations.

For this reason, some researchers say that the benefit of antigen tests’ rapid results outweighs the cost of these tests’ lower accuracy.

The modeling study backs this up. In one analysis, the researchers found that, if it takes more than two days for people to get their PCR test results, the results for reducing infections are similar to an antigen test-only scenario.

And if PCR test results take more than five days, the results are worse than an antigen test-only scenario.

The researchers say that low- and middle-income countries unable to buy tons of expensive PCR tests should rely on regular antigen testing to get outbreaks under control.

This could be a critical strategy as the Delta variant spreads in many less-wealthy nations around the world.

‘Tests are continually improving, and the tradeoffs are in favor of rapid testing, even if it is less sensitive,’ said Gautam Menon, professor of physics and biology at Ashoka University in Sonipat, India and one of the study’s lead authors.

‘Modeling the effects of using different combinations of tests, keeping in mind their relative costs, can suggest specific policy changes that will have a substantial effect on changing the trajectory of the epidemic.’ 

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