By Michael Le Page
Taechit Taechamanodom/Getty Images
There are many antibody tests available that can reveal if you have had and recovered from the coronavirus. Is it worth paying for one of these tests?
What can a coronavirus test tell me?
There are two main kinds of tests for coronavirus. Which one you take may depend on what you want to find out: do you want to know if you are infected now, or have been in the past?
One type of test looks for the virus in swabs of the nose or throat, which can reveal if you are currently infected. In many countries, tests for active infections are free, so there is no need to pay.
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The other type of test looks in your blood for the antibodies that your immune system makes to attack the coronavirus. This can reveal if you were infected but have since recovered. It can take several weeks for antibodies to be produced, so there is little point in doing antibody tests during or soon after an illness that you suspect could be covid-19.
Can an antibody test tell me if I’m immune to the coronavirus?
No, it cannot. “A positive result may not mean a person is immune,” says the UK’s Medicines and Healthcare products Regulatory Agency (MHRA). People who have recovered should be immune for a while at least, because their immune systems have successfully eliminated the virus.
But we don’t know how long immunity against the covid-19 coronavirus lasts yet. Studies of other coronaviruses that already circulate in humans show that people can be reinfected as soon as six months after the initial infection.
What use are antibody tests?
For policy-makers, it is useful to know what percentage of a population has been infected. For individuals, until we find out more about immunity to the coronavirus, antibody tests are less immediately useful.
You shouldn’t alter your behaviour on the basis of a positive antibody test, because this will increase your risk of becoming infected again and you could then go on to infect others. Plus, your test results might not be correct.
I’m still curious. How do I get an antibody test?
In the UK, your only option is to have a blood sample taken with a needle from your arm by a qualified healthcare worker and then send your sample off for testing. Some private doctors and clinics will send someone to your home to take the blood sample.
Aren’t there home testing kits available?
The antibody tests that are designed to be done entirely at home haven’t proved reliable so far. In the UK, it is illegal to sell them but there are reports that some companies are doing so. “We strongly discourage organisations and individuals from purchasing unvalidated antibody tests,” says a spokesperson for the UK’s Department of Health and Social Care.
Can I take a blood sample myself?
Some companies were sending out self-testing kits that involve pricking a finger and placing a drop of blood in a narrow glass tube. However, on 29 May, the MHRA ordered companies to stop until it has been shown that the antibody tests work with blood samples taken this way.
Which antibody test is best?
So far, 219 antibody tests have got a CE certification mark, which means they can legally be offered in the UK and Europe. But having a CE mark doesn’t mean there has been any independent validation of these tests. “There’s no scrutiny,” says Jon Deeks of the University of Birmingham, UK.
What’s more, Deeks says many of the clinics offering antibody testing don’t reveal which specific test they use. Many countries, including the US and Australia, have stricter regulations.
Do we know how accurate any of the tests are?
Public Health England has independently evaluated five tests developed by Roche, Abbott, Euroimmun, DiaSorin and Ortho Clinical. These tests have a sensitivity after about 14 days of infection of around 70 per cent or higher – meaning 30 per cent or fewer results are false negatives and wrongly identified as not having the virus. These tests also have a specificity of about 98 per cent or more – meaning less than 2 per cent of results are false positives and wrongly identified as having had the coronavirus.
If I get a positive result from one of these tests how certain can I be that it’s correct?
The predictive value of tests matters more than the sensitivity or specificity. The predictive value depends on your likelihood of having been infected, says Babak Javid of Tsinghua University in Beijing.
So if you live in London and have recently had a severe respiratory infection, a positive result is likely correct. But if you have been shielding in Cornwall with no symptoms, it could well be wrong, says Javid.
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