Roberto Pfeil/dpa
How do you test for coronavirus infections?
At present, most tests are based on looking for genetic sequences specific to the covid-19 coronavirus. If these sequences are found in a sample, it must contain the virus.
What does testing involve?
Getting a sample to test involves pushing a swab – which resembles an extra-long cotton bud – deep inside the nose or to the back of the throat. The swab is then sent off to a lab.
What about testing blood or urine?
The virus is only detected in the blood, urine or faeces of roughly half of those who test positive based on nose or throat swabs, so blood, urine and stool tests aren’t reliable. If you are coughing up sputum, testing that can provide more accurate results than a nose or throat swab, according to a handbook summarising findings in China.
How long does it take to get a result?
Most labs use a method called the polymerase chain reaction (PCR), which takes several hours. It can take days for labs to run the tests and tell people their result. Several groups around the world, are developing faster genetic tests, typically based on a method called loop mediated isothermal amplification (LAMP), which takes less than half an hour. Handheld LAMP tests that could be used in homes and airports may start to become available within weeks.
How accurate are the tests?
In theory, genetic tests should be extremely accurate if done properly. However, there have been reports from China of many false negatives and false positives. This may be because the swabbing wasn’t done correctly, or because overworked lab technicians were making mistakes. In addition, if people are tested very soon after becoming infected, they may not be shedding the virus yet.
Why is it so hard to get tested in most countries?
There are obvious practical issues with scaling up testing, from lack of trained personnel to equipment. But South Korea, which is now testing more than 20,000 people per day, has shown how fast it can be done. Many other countries didn’t start ramping up testing capacity until local case numbers began to soar and – unlike South Korea – haven’t made testing central to their strategy as advised by the World Health Organization.
I have heard some tests can be done in 10 or 15 minutes. How do they work?
Rapid tests, such as the one that may roll out in the UK soon, are usually based on detecting proteins rather than genetic sequences. These proteins can either be viral ones, called antigens, or the antibodies our bodies make to kill the virus. Antigen tests can directly detect the presence of the virus, but are less accurate than genetic tests.
What about antibody tests?
The downside of antibody tests is that they cannot detect infections in the first two weeks or so, when people are most contagious. However, our bodies keep making antibodies even after we have recovered from an infection, so testing people’s blood for antibodies against the coronavirus will reveal how many of us have been infected so far. This will help us calculate the infection fatality rate.
Can antibody tests distinguish between people who have recovered and those who are still infected?
Sometimes. People start producing so-called IgM antibodies against the coronavirus around 10 days after showing symptoms (perhaps 15 days after infection). After another two days, their bodies start making IgG antibodies, and gradually stop making IgM. Most people will recover fully as soon as IgG levels ramp up. Many rapid tests can detect both types of antibodies. If IgM antibodies are present in someone’s blood, they are likely to be still infected. If only IgG is present, they are recovering or fully recovered.
How accurate are these rapid tests?
We don’t know. Numerous companies are producing different tests that haven’t been independently checked yet. In general, testing for antigens or antibodies is less accurate than genetic testing, but the tests are easier and cheaper to manufacture. Rapid tests could play a valuable role, especially in poorer countries with little testing capacity. If they can reliably tell us when people have already been infected, it would allow key workers – especially in healthcare – to continue working without worrying about becoming infected or infecting their families.
Read more: https://www.newscientist.com/article/2238477-how-does-coronavirus-testing-work-and-will-we-have-a-home-test-soon/
