On testing again

Testing has been covered on this blog several times, for example here on true and false positives, and here on interpreting data, and here on a statement by a former US president. As the cases generally fall down in many places in the world, we start worrying about false positives and false negatives.

I have just written another piece for The Conversation, explaining why – as the numbers drop – we might be seeing more false positive test results than true positive ones. This is a normal thing – and not a reflection on the quality of the tests (and nothing to do with PCR “cycle” numbers).

Also, pointing out that with 50% sensitivity we are likely to see a lot of false negatives, each one of which might trigger an outbreak. This is potentially more worrying and shows why we need to be very cautious to build the whole strategy of school reopening on Lateral Flow Tests.

This is not to denigrate the importance of testing, but more to explain why the test results need to be interpreted cautiously. Tests, together with tracing – and support for those self-isolating – are a key to #ZeroCovid strategy.

But, as with every strategy, we need to use them carefully and Reverend Bayes clearly has a lesson for us in this respect (pity I could not mention him in the piece for lack of space).