Tests are the key to unlocking lockdown but how accurate are they?

By Jerome Burne

Distributing tests as fast as possible to discover who is now infected with the virus, has become a test of organisational machismo for the government – 100,000 promised by the end of last month. Apparently they hit the target but only by sleight of hand with the figures – something this government seems not averse to. 

But what we really need to know is whether the tests are any good. Tests are an essential and valuable part of medicine. They come in a reassuring yes/no format – I’m afraid you do have HIV, no you are not pregnant. 

But there is also a long history of disputed tests such as the mammogram. Apparently, a valuable use of technology to spot breast cancer early, it may damage more patients with unnecessary treatment than the number of cancers prevented.  Unreliable tests that generate too many false positives (reporting you have it when you don’t) or false negatives (saying don’t when you do) can be worse than nothing. 

The tests to tell if you have had the virus, and so are probably both immune and non-infectious, will rely on detecting the antibodies in the blood, that the body makes in response to infection. But it turns out that the antibodies to Covid 19 work in ways that are still mysterious. 

For example, even when you have them, it’s not clear how long you could be shedding the virus or how long they protect you or crucially, whether you could be re-infected.  (more…)