COVID-19 tests

Executive summary

* The key message from the World Health Organization about COVID-19 has been "test, test, test."

* While much is being said about testing, less is being said about test reliability.

* There are two key attributes of a medical test – test sensitivity and test specificity. Test sensitivity is the ability of a test to correctly identify those with the disease. Test specificity is the ability of a test to correctly identify those without the disease.

* There are two tests related to COVID-19 that would be extremely useful right now: Test A) Knowing who has the disease (present). Test B) Knowing who has had the disease (past).

* Test A needs to be prepared to diagnose some people with COVID-19 who don't have it – false positives – to err on the side of caution. Test B needs to be prepared to miss some people who have had COVID-19 – false negatives – to err on the side of caution.

* There is very little evidence on the reliability of Test A. No government in the world has yet rolled out a full programme for Test B.

* “Test, test, test” is the right strategy in principle, but, in practice, test limitations need to be accounted for and other options need to be explored.

Brief update on previous note

Things have moved at quite a pace on masks since my last note. On March 31st, the World Health Organization (WHO) stood by its recommendations that members of the public should not be wearing masks (Ref 1). On April 2nd, news stations around the world reported that the WHO was reviewing the evidence (Ref 2). In the early hours of April 4th, the South China Morning Post reported that the WHO had changed course and now recommended wearing masks in public (Ref 3).

The US Centers for Disease Control and Prevention (CDC) has just issued new advice to wear cloth masks (Ref 4). At the time of writing this, the Deputy Chief Medical Officer for England said that the advice (to not wear a mask) had not changed (Ref 5).

Introduction to this week’s note

Testing is the other hot topic in COVID-19 news. The key message from the WHO conference on March 16th, 2020 was “test, test, test” (Ref 6). This makes sense. We are making huge decisions at the moment with little information and the more information we can gather, the better. However, all medical tests have limitations and we need to be aware of these limitations in the context of COVID-19. This note, therefore, is about the limitations of medical tests generally, what we would like to know about COVID-19 tests and what we may not be able to know for sure.


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