There has been a lot of noise about false positive COVID19 tests in the news. So, I thought I would try to explain what it all means. Or do my best anyway.
There are two measures in most medical screening tests which are usually defined as the ‘sensitivity’ and the ‘specificity’ of a test. In my opinion, these two words are far too close together in sound, so they are very easy to mix up in your brain.
I find it easier to think of the accuracy of test results in this way.
- False negatives
- False positives
A false negative is a result which informs someone that they do not have a disease, when in fact they do.
A false positive is a result which informs someone they do have a disease, when they don’t.
Ideally a test should never give a false negative (100% sensitivity) nor give a false positive (100% specificity). There is no known test that does this. In general, there is a trade-off going on between these two measures.
By which I mean, if you aim for 100% sensitivity, the specificity often falls away – and vice-versa
For example, in cancer screening the primary objective is you must never miss a case. So, the sensitivity rate is set very high. By definition the rate of false negatives is very low.
A shadow on the breast, a few strange cells here, a few strange cells there – ‘that might be cancer, better to be safe than sorry. Don’t take the risk’. Positive cancer test.
To put this another way. The fear of missing any cases of cancer results in the number of false positives being high. This raises the question with COVID19. Is it better to underdiagnose – many false negatives. Or over diagnose – many false positives?
Note I am talking here primarily about the naso-pharyngeal swab tests (i.e., antigen tests) which are used to see if you have the virus NOW and not the blood (antibody) test done which may be done later to see if you have ever had the virus.
This issue does not seem to have been discussed. If you want to prevent spread of COVID19, you would presumably want very few false negatives in these swab tests. Otherwise people will be told they don’t have the disease – when they do – and happily go off spreading it around. Equally, you would be relaxed about false positives. People would isolate when they don’t need to, but not a great health issue.
Weirdly, however, this does not seem to be the case.
COVID19 false negatives
With COVID19, there are a lot of false negatives, with some studies quoting figures as high as 50%. That is, half of those told they are not infected with COVID19, are probably infected1. A systematic review got figures between 2% and 29%. So, we could call that an average of 16%?
As you can see, these figures are clearly all over the place. This is in major part because there is no ‘gold-standard’ COVID19 test. By which I mean that we do not have a ‘test of tests.’ Namely, the expensive and time-consuming test by which we absolutely can know if someone truly is infected. The test against which your ‘field tests’ can be calibrated/verified.
Indeed, currently, there is no current agreement as to what ‘infected’ means with COVID19. Does it mean finding viral particles in the nose, sputum, or throat – or all three? Does it mean finding viral particles in these places, and also isolating it in the bloodstream, or lungs? Does it mean finding evidence of antibodies specific to COVID19 two to three weeks following ‘infection?’ Or what? It would be nice to know.
COVID19 false positives…
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