We know testing is all important to get through this. People need to know if they have active virus so they can adjust their behavior accordingly. Anyone who tests positive should self quarantine for 14 days and we also need to do contact tracing to find the people that they have been near to. (I feel that forced quarantine in one of those many nice empty hotel rooms at government expense is the way to go for people who won’t (or can’t) self quarantine. My understanding of quarantine law is that this is a permitted use of governmental power. I know Singapore is doing this. Heck it would even help the hotel industry-which is slowly dying.)
Next, if having (enough) antibodies to Covid 19 means you can’t infect people any more, that would be a game changer. This means accurate testing for antibodies could become really important. As I write this we don’t actually know if having enough antibodies means you no longer can spread the disease. So at this point, antibody testing just tells us what percentage of the population had Covid 19 but were never diagnosed. This is certainly useful information but nowhere as important as knowing if you can’t spread the disease would be. And as I write this, there are way too many crappy antibody tests out there because the FDA sure screwed up at first. Thankfully, they are now trying to fix that (https://www.fda.gov/news-events/fda-voices/insight-fdas-revised-policy-antibody-tests-prioritizing-access-and-accuracy).
But testing can’t be perfect. As the old engineering joke has it: you always tell the customer that they can have it fast, cheap or good. They need to pick the two they really want. In medical testing you also get to choose from three options: how fast you want the result is one but more importantly you almost always have to have to choose to optimize your testing for false positives or false negatives. A false positive means, naturally enough, the test says you are infected, but you are really not. A false negative is the opposite: it says you are not infected but you are. Note: while almost no medical test can be optimized for both-at least not at the start of the disease, it is not impossible (eventually). For example, current HIV tests have almost no false positives or false negatives.
So what are you really interested in? Well the percentages of false positives and negatives of course. For a diagnostic test, in a situation like Covid 19, you want a very small percentage of false negatives. You don’t want to tell someone they don’t have the disease when they really do. In testing speak, you want a very sensitive test. Negative results from highly sensitive tests are very useful. (Interestingly enough, a positive result from a very sensitive test is often less useful. More on that in my next blog entry.)
So, suppose for example, you have a test which, when you give it to a 100 people who are infected, it only reports 85% of them as having the disease. We say that the test is 85% sensitive and has a 15% false negative rate. I think we can agree that is not a great diagnostic test. By way of contrast. a single modern AIDs test is 99.5% sensitive and one can do multiple independent tests if you want to increase accuracy. The best home pregnancy test is at least 95% sensitive and probably a bit higher than that when used correctly. Highly sensitive tests are quite common.
Let’s go back to our not so great 85% sensitive (15% false negative) test. Guess what? The Abbott ID NOW test, which has created lots of excitement because it can get results in less than 15 minutes (and is used by the White House to do daily testing of white house employees now ) is about 85% sensitive-and that is actually pretty good for a Covid 19 test. (Well 85.2% https://www.npr.org/sections/health-shots/2020/04/21/838794281/study-raises-questions-about-false-negatives-from-quick-covid-19-test). More generally, the most used tests for Covid 19 are basically crap, they all have lousy sensitivity. They are plagued by false negatives.
(Interestingly enough, many of the tests are quite sensitive when done under laboratory conditions, it’s how they are done in the field that makes them insensitive. The main problem may simply be that any test that relies on sticking a swab way up a person’s nose may not be done right in the field. An MD friend of mine says that having a red line near the end of the swab would be a cheap low tech solution to improve accuracy! Turns out nasal swabs are not easy to use correctly.)
A saliva test works much better in field conditions, people rarely screw up when asked to spit into a tube. And they are beginning to come out, in particular the Rutgers test (https://www.rutgers.edu/news/new-rutgers-saliva-test-coronavirus-gets-fda-approval) looks to be a breakthrough because it is both sensitive and easy to administer. Alas (see the rules for optimization), it seems to take a minimum of 24 hours to get a result.