Will antibody tests for the coronavirus really change everything?

Touted as society’s way out of widespread lockdowns, scientists say the true potential of these rapidly-developed tests is still unknown.

British Prime Minister Boris Johnson called them a ‘game changer’. Antibody tests have captured the world’s attention for their potential to help life return to normal by revealing who has been exposed, and might now be immune, to the new coronavirus. …

Flood of tests

When a virus invades the body, the immune system produces antibodies to fight it. Kits detect the presence of antibodies using components from the virus, known as antigens. Tests generally fall into one of two categories: lab tests that need to be processed by trained technicians and take about a day, and point-of-care tests that give rapid, on-the-spot results within 15 minutes to half an hour. Several companies, including Premier Biotech in the United States and China-based Autobio Diagnostics, offer point-of-care kits, which are designed to be used by health professionals to check if an individual has had the virus — but some companies market them for people to use at home. …

Test the tests

One problem, however, is that most kits have not undergone rigorous testing to ensure they’re reliable, says Busch. During a meeting at the UK Parliament’s House of Commons Science and Technology Select Committee on 8 April, Kathy Hall, the director of the testing strategy for COVID-19, said that no country appeared to have a validated antibody test that can accurately determine whether an individual has had COVID-19.

Kits need to be trialled on large groups of people to verify their accuracy: hundreds of people who have had COVID-19, and hundreds of people who haven’t, says Peter Collignon, a physician and laboratory microbiologist at Australian National University in Canberra. But so far, most test assessments have involved only some tens of individuals because they have been developed quickly.

It seems that many tests available now are not accurate enough at identifying people who have had the disease, a property called test sensitivity, and those who haven’t been infected, known as test specificity. A high-quality test should achieve 99% or more sensitivity and specificity, adds Collignon. That means that testing should turn up only about 1 false positive and 1 false negative for every 100 true positive and true negative results.

But some commercial antibody tests have recorded specificities as low as 40% early in the infection. In an analysis2 of 9 commercial tests available in Denmark, 3 lab-based tests had sensitivities ranging 67–93% and specificities of 93–100%. In the same study, five out of six point-of-care tests had sensitivities ranging 80–93%, and 80-100% specificity, but some kits were tested on fewer than 30 people. Testing was suspended for one kit. Overall, the sensitivity of all the tests improved over time, with the highest sensitivity recorded two weeks after symptoms first appeared. Some of these tests are also being used to test individuals in other countries, including Germany and Australia.

Point-of-care tests are even less reliable than tests being used in labs, adds Smith. This is because they use a smaller sample of blood — typically from a finger prick — and are conducted in a less controlled environment than a lab, which can affect their performance. They should be used with caution, he says. The WHO recommends that point-of-care tests only be used for research.

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