The Limits of Testing

In her most recent Washington Post column Megan McArdle makes some good points about testing. In reference to the Abbott Labs test that the White House was using and other rapid testing methods:

The high rate of false negatives means that testing provides the most protection when it’s deployed at the population level. At the group level, it’s only a weak, adjunct tactic to other precautions. And at the individual level, it’s borderline useless.

Start with the individuals: If you think you might have been exposed to covid-19 — for example, by flying home for the holiday — a negative test can’t tell you it’s safe to hang out indoors with your elderly parents. There’s a significant risk that you simply aren’t testing positive yet, and with case fatality rates still extremely high among older age groups, that’s a risk you should take very seriously.

That’s the reason I’ve been so skeptical about EZ home testing as a way back to normal. It’s not nearly as useful for that purpose as its exponents seem to think. Ms. McArdle continues:

At the group level, we should think of testing the way medieval kings thought about fending off barbarians. They didn’t just throw up a wall and call it a day, because what do you do if the enemy breaches the wall? So they built a layered defense — moats, catapults, boiling oil — plus some soldiers inside the walls. That’s also the best way to hold covid-19 at bay: testing and masks and hand-washing and distancing and good ventilation (or best of all, staying outdoors). Because if you instead try to use testing as a substitute for other safety measures, then eventually the virus is likely to slip through your lone line of defense and wreak mayhem within.

I’m not nearly as optimistic about the prospects for mass testing as she is:

If those people who test positive do the patriotic thing and immediately quarantine themselves, then in the second round of transmission, we would end up with 30,000 infections, instead of 300,000. In the third round, having caught 27,000 of the second round of infections, we end up with 9,000 new infections, instead of 900,000. Repeat that a few more times, and we’ve basically wiped out the novel coronavirus without doing much else.

The problem, of course, is that I think that relatively few will “do the patriotic thing” which renders the whole exercise useless. What I have been saying for something like nine months now is that what we really need is epidemiological testing and, honestly, it might be too late for that. It would have enabled policy-makers to craft more narrowly-tailored mitigation strategies. I write this as Chicago shambles to a stay-at-home directive like the one that was in place in March. The difference is that March was March and it’s December now.