Some stats on the importance of accuracy of antibody tests for Coronavirus:

1/7 Let us say that a test of whether you have had the Coronavirus, and have built up immunity, or have not had it, and have no immunity, is 90% accurate.

2/7 By which I mean there is a 10% chance it says you’ve had it when you haven’t (a false positive) and a 10% chance that it says you haven’t had it when in fact you have and are immune (a false negative).

3/7 Now if current estimates are correct and the mortality rate of covid19 is ~1% and there have been 33,000 deaths in the uk, this means there have been 3.3 million infections thus far, which is 5% of the population (in line with estimates in France and Spain).

4/7 So let’s say you take the antibody test and it says you’ve had the virus and are now immune. What’s the chances this is right? Is it 90%? No.

5/7 If you are one of the 3.3 million who have had it then you would be one of 3 million (90%) who would correctly show positive and have immunity.

6/7 But if you are one of the 63 million who haven’t had it then you could be one of the 6.3 million (10%) who show a false positive - that is the test mistakenly says you have had the virus.

7/7 This means if you were to take that test today and it says you’ve had the virus, it is twice as likely to be wrong than right.

So antibody tests really have to be close to 100% accurate to be of use.