This is a tweet thread about rapid testing. It is meant to highlight something that most people, including most journalists and doctors, who are publicly skeptical about rapid tests seem to miss.

Why is a philosopher weighing in on this? My research is on systems...

The proximate cause of this thread is this article from the NYT suggests that we should be skeptical about rapid tests at home.

The article captures all the mistakes of lots of thinking about rapid tests. https://www.nytimes.com/2020/09/06/health/coronavirus-rapid-test.html

First, people who propose rapid tests, like @michaelmina_lab, are proposing that we institute a new *testing system* not just that we drop paper strip rapid tests from a plane and hope that everything goes well.

A rapid test system has three legs:

1. The test technology itself (the paper strips);
2. How widespread the use is of that technology;
3. How people respond to the use of that technology.

Any assessment of a testing system should compare it by considering all three legs.

This system would supplement the PCR-based testing system, with aim being to lower the demand for that system's use, while increasing overall surveillance capacities.

The individual rapid test's lower sensitivity and specificity is not in itself a problem since we need to assess how sensitive the testing system is in concert with PCR testing system. We also shouldn't think of the rapid test as a one-off test, but instead as taken serially.

That means that the rapid test *system* in concert with a PCR testing system will actually have high sensitivity and specificity - and in fact will be MUCH more sensitive and specific than the PCR system alone.

Next to how widespread the use will be.

Rapid tests take huge infrastructure required for mass PCR testing and socialize it to allow more tests. Instead of trained health technicians and expensive labs running tests, factories print and govt distributes paper tests.

We have increasing evidence that when people have access to regular testing, they get tested and they in fact respond to positive tests by restricting behavior in a way that prevents spread of infection. Let's make this universal. https://www.newsobserver.com/news/local/education/article245382380.html

Finally, how will people respond? Many people who have been exposed or have a cold or slight fever don't get PCR tested.

This is because PCR testing is onerous: you have to make an appointment, drive to the appointment, often pay for the test, and then wait days for results.

This means that many people go undiagnosed as infected or do not quarantine while waiting for test results.

Mass paper testing ensures that people who have been exposed or have symptoms test themselves. There are very low barriers to testing if the test is simple and at home.

What will people do then? Most people, if positive, will want to protect family and friends. They will probably call a doctor and quarantine.

Even better, though, would be if PCR testing, because there is less pressure on it, becomes cheap and easy.

People with positive paper strip tests can easily get PCR tested, get quick, accurate results.

Their results will be reported to the state and they can both quarantine and get necessary care.

Rapid tests are a system, not a thing. They are an addition, not a substitution. END