So, I haven’t gone to Chontal yet, and won’t go until at least next Wednesday, which is Ash Wednesday.
On Monday morning, after arriving from Mindo, I took a Covid PCR test here in Quito. Although I didn’t have symptoms, I took the test out of courtesy to the community in Chontal, who would be hosting me for a few months. Since my last test, I had traveled from Boston to Quito to Mindo and back to Quito, so that would give folks some consternation.
Well, the test came back positive on Wednesday! They don’t do followup here like in the States, so I decided to go and get another one right away from another lab. That one came back negative! Which basically put me back to square one before doing any testing at all.
Why do I say that?
Well, come on for a technical ride as I put on my R&D engineering hat ….
Before, I had done some research on Covid data and testing in anticipation of this testing, wondering what was worth doing for testing. I put together a model that includes elements of other models I found on reputable sites the internet, and accounts for latest estimates of PCR testing sensitivity and sensibility (measures of how well it detects positive and negative cases); the local covid data and its underestimate of the virus’ prevalence; my locations; and the sequential statistical nature of repeated tests. It includes some conservative elements: it doesn’t account for reduced “positive” probabilities due to my lack of symptoms, nor for the higher possibility of poorer lab control and positive specimen contamination here in Ecuador. However, non-conservatively, it doesn't account for other problems that can cause false negatives: poor specimen collection or the possible presence of inhibitors to the test in specimens. (These are things that professionals in the field note.)
Anyway, when I tested positive the first time, I estimated roughly 80-85% probability of having Covid. I decided to get another test from a different lab: another positive test would have put me at pretty much 100%. But it turned out negative, putting me statistically at …. about 50-50% ! Basically in the "unknown" range of someone who has traveled - pretty much back to before having been tested in the first place (though $125 poorer). Which is to say, I should presume to be carrying it and quarantine and watch for symptoms, which I'm doing.
First of all, for me, it shows why the CDC wouldn’t recommend a second PCR test right after a first positive one. Of course, another positive result wouldn't change the course of action. But a negative second test, although producing more doubt, doesn't actually change the course of action either. In fact, after a second negative test here in my situation, you'd need 2 *more* negative tests in succession to cross the 95% probability threshold of being Covid-free!
Now, if I had tested negative in the first test, I could be 99% confident of being covid-free, as long as there was confidence in the collection & testing, and I wasn't around any Covid "cluster" or in prolonged close contact with someone with Covid. So, there's some confusion: people can think that if they get a negative after a positive, they can carry that 99% certainty. But you can't. It doesn't cancel out the positive. If it comes after a positive result, it's a different thing statistically than coming in the first test. I say that because people can quickly assume that "The first must have been a false positive," and that they're definitely negative. This can affect expectations of behavior (as I've noticed !) This confusion is all the more reason not to encourage a second PCR test right after a positive result!
So, anyway, since I have a safe and convenient place to stay here in Quito for now, with much thanks to Rosa and the folks who are staying here. I’m planning on staying in a form of quarantine here until next Wednesday or Thursday, unless something serious unexpectedly comes up health-wise. I still have no symptoms of any kind, and feel great. It’s all costing me more than I had planned on spending so far, but it’s better to do things conservatively out of respect for everyone and their safety.
I'll aim to send an update next week sometime. Have a blessed weekend, Valentine's Day, and week (it's the Carnaval holiday this weekend here.)
In my room in Quito, by the window where I get my food served :)