Saturday 18 April 2020

Day 33 of self-isolation - testing


Testing is going to be really important to ease the lock down. But testing isn't as simple as it sounds.

When you do a medical test, there are false positives and false negatives. A false positive is when a test says that you're infected, but actually you are not. A false negative is when the test says you're clean, but you're infected. I know all about false positives and false negatives from my previous experiences.

Medical tests are rarely without false positives, false negatives, or both. Clearly, a test with fewer test errors is better than one with more test errors. But if a test is 99% accurate, is that better or worse than a test that is 95% accurate, but a tenth of the cost (meaning that you can  test ten times as many people?)

And when you have a test, how do you assess how many false positives and how many false negatives? For that, you need to compare it with a "gold standard" test, which is 100% accurate. And I doubt if we'll have one. So the accuracy of testing, will be a bit of a guess.


Everyone now is focusing on testing. But please remember that these tests won't be infallible, and that we probably won't have an accurate idea of how fallible they are.

Yes, I know. Probability, quality control and statistics are a mess. That's how the world is.


The UK spent £16 million on tests from China, a few weeks back. That was the basis for our government telling up that testing would be greatly increased. We paid upfront for that, sight unseen. We sent them to Oxford University for evaluation, and they turned out to be useless for what we wanted. They were tests for antibodies in people who were already heavily infected. That doesn't tell you if someone is in the first week of infection and hasn't generated antibodies yet.

That's what happens when a government panic-buys.

So now we have £16 million worth of tests that don't do what we need, and a big shortage of masks, face shields and gowns, all of which is very low tech, and I would have thought that it wouldn't be beyond the wit of our government, to find people who could manufacture large numbers of these.

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