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RT-PCR tests for the coronavirus are very accurate but required skilled technicians and must be done in a laboratory. Photo: Bloomberg

Explainer | Could a coronavirus test be as simple and fast as a pregnancy test?

  • Without a vaccine, quicker diagnosis and quarantine could be the answer to stopping the pandemic, some scientists argue
  • Various technologies are in development but the question is how accurate do they need to be?
Disease
While much of the world’s hopes of overcoming the coronavirus epidemic are on a vaccine, the race is also on to develop a rapid test to detect the pathogen.

Scientists said simple, cheap, at-home kits like pregnancy test sticks could allow people to be tested more often and put in quarantine sooner, making it safer for schools and public activities like football matches to resume, especially before vaccines are available.

Hundreds of different kinds of diagnostic tests are in development and a few have been approved for emergency use.
Conventional RT-PCR tests are still the most accurate but they require expertise and equipment. The debate now is whether some accuracy in testing should be sacrificed for speed and frequency to stop the spread of the coronavirus.

What are PCR tests?

A polymerase chain reaction (PCR) test is a kind of molecular test that detect the genetic material of the coronavirus in a specimen.

The gold standard for diagnosing Covid-19 is the RT-PCR test.

In this test, samples from swabs or deep throat saliva collection are put through a series of heating and cooling cycles to amplify the viral RNA for detection. The process usually takes several hours and requires expensive laboratory equipment. Some companies have cut the time for a result down to 75 minutes but typically it still takes a day or two from specimen collection to results.

These tests are very sensitive and can detect almost all of those infected with the coronavirus, even if the amount of virus in the specimen is very small.

The downside is that they have to be carried out in laboratories, which is expensive and requires trained personnel. In many countries, demand for tests has swamped testing capacity, delaying quarantine of infected people and containment of the coronavirus.

03:07

Hong Kong’s Covid-19 mass testing ends with at least 42 carriers found among 1.78 million people

Hong Kong’s Covid-19 mass testing ends with at least 42 carriers found among 1.78 million people

Is there a cheaper, quicker test?

Another option is an antigen test, which detects specific proteins on the surface of the coronavirus.

This kind of test is considered less accurate than a molecular one but doesn’t need to be done in a laboratory and can have a result in 15 minutes. It is also cheaper.

Professor Adrian Esterman, chair of biostatistics at the University of South Australia Clinical and Health Sciences, said antigen tests could be done quickly, and “anyone found positive immediately isolated and contact tracing undertaken”.

“This is incredibly important in reducing transmission of the disease. One or two days can mean the difference between controlling the outbreak, or it getting out of hand,” Esterman said.

There are various types and the most common one uses antibodies on a lateral-flow strip which contains antibodies that bind to specific viral proteins and turn a colour when the virus is present.

These tests can miss many positive cases but the ones they do pick up are rarely false positives, especially when the coronavirus is prevalent. But the number of false positive is higher in areas of low prevalence.

They also require a lot of virus in the specimens for detection and tend to work best when the samples are taken when the person’s viral load is highest – within the couple of days before or about a week after a person shows symptoms.

“They are not as sensitive as the RT-PCR test, with sensitivity ranging from 50-90 per cent. At the lowest end, one in two infected people might be told that they do not have the disease,” Esterman said.

Antigen tests for coronavirus are becoming more common in India. Photo: EPA-EFE

Why use antigen tests if they are less accurate?

Antigen tests are becoming more common in countries like the United States and India.

In India, 25-30 per cent of the country’s confirmed cases are detected by antigen tests. But some research shows that one of the antigen test kits approved for emergency use in the country could miss half of those infected.

In its guidelines, the World Health Organization said antigen tests should be able to pick up at least 80 per cent of the positive cases.

An adviser on the guidelines, Rosanna Peeling, chair of diagnostics research at the London School of Hygiene and Tropical Medicine, said that even if the tests missed 20 per cent of the positive cases, they were still very useful to stop transmissions because most of those being picked up were at their most infectious.

“It is not that you randomly miss 20 per cent. The 20 per cent are the people with low viral load, low amount of virus and therefore may not be as important as transmission risk as the people who are antigen positive,” Peeling said.

“If you think about the risk and benefit, the harm of not using an antigen test and therefore let many people who may be virus positive go around to transmit. That is worse than a false negative.”

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Global coronavirus cases surpass 25 million as India deals with world’s fastest-growing outbreak

Global coronavirus cases surpass 25 million as India deals with world’s fastest-growing outbreak

How good are some of the antigen tests?

The US Food and Drug Administration has approved four antigen tests for emergency diagnostic use.

According to the developers, their sensitivity ranges from 84 to 98 per cent.

Most tests still require a machine to read the results but in late August, the FDA approved a kit developed by Abbott Laboratories that shows the result on a card. It is similar in design to a pregnancy test, gives a result in 15 minutes and costs US$5 per test. The company says it picks up 97 per cent of cases.

US industrial conglomerate 3M and MIT are also developing a rapid antigen test that they hope will give a result within a in a few minutes on a “paper-based device”.

Antigen tests are quicker than molecular tests but they are also less accurate. Photo: EPA-EFE

Can we get one now?

Supplies are limited. The US government has placed a US$750 million order with Abbott for 150 million kits.

And these tests are not home tests – health professionals still need to collect the samples and read the results. Nevertheless, it is a step forward because the tests can be done in schools, clinics, and homes for the elderly.

Could over-the-counter tests make a difference?

One of the most prominent advocates for over-the-counter antigen tests is Michael Mina, assistant professor of epidemiology at the Harvard T.H. Chan School of Public Health.

Mina argues that antigen tests should be a surveillance tool rather than a diagnostic one and so in household use.

In a transcript of a press conference on the school’s website, Mina said the benchmarks for these tests should be lower because frequent and rapid testing was more important in cutting off transmissions than the sensitivity of tests.

Mina said the approved antigen tests were like espresso coffee, requiring a machine, when “instant coffee” home tests could really stop the spread of the coronavirus.

He also said the stringent regulatory requirements were slowing down the development of rapid tests.

“[Developers] are trying to spend more time and more money to better and better optimise the tests, which might take months so that they can meet the FDA approval,” he said.

What other technology is out there?

Many scientists are working on various technologies to create home tests.

One of them is loop-mediated isothermal amplification (Lamp) which uses a constant temperature to amplify RNA in samples.

It doesn’t require the repeated cycles of heating and cooling that the PCR test needs but it is complex and has to be done in a lab, according to Peeling.

“It has been around since the 80s. It was first developed by people in Japan and the [tuberculous] programme invested a lot of money in Lamp but still did not come out with commercialised tests. I would be a little bit cautious throwing my hat for the Lamp test,” she said.

Other approaches use the gene-editing technology Crispr, nano technologies to detect the virus in breath, and microscopes to detect the virus visually.

Peeling said it was unlikely any of these technologies can be commercialised soon.

“The reality is that from bench-proof principle to commercialisation takes years,” she said.

“I will be very surprised a lot of these new technologies are able to be commercialised.”

Peeling said so far antigen tests were the most promising candidates to become rapid home tests.

“I am looking forward to the day the simple strip test can be used everywhere and people could produce millions of them in a week. Those are the ones that will really be a game changer.”

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