Testing, Testing: How and When to Test for Covid-19
Let’s talk about Covid Testing.
First off, let’s be clear: why do we do testing? To help treat sick people, yes, but a huge part of the reason is to keep contagious people at home.
The science of testing has improved greatly. As a reminder, there’s two general kinds of tests:
swab (for virus, to show if you HAVE the virus, ie, to see if you are infected and contagious)
blood (for antibodies, to show if you HAD the virus in your body).
Two caveats about antibody testing:
a) this is not the right test to see if you are currently contagious
b) some early tests were super lousy, the current tests are much better
But: no medical tests are perfect. Ever. Mammograms have false negatives. Colonoscopies can have false negatives. You can even have false positives for syphilis! (Good luck explaining that to your spouse, though.)
False negatives in Covid — where the test is negative but you actually do have Covid — are a vexing issue because of the nature of viruses: they take a while to build up in our bodies so the test can’t detect them early on. It turns out you actually need to wait a bit to get the highest chance of an accurate test.
Your “best” results (highest likelihood of getting an accurately positive test) is 5–8 days after exposure or around day three of symptoms.
I learned very quickly in med school that context is everything; the best way we learn is by hearing examples (or “cases” as we call them in medical practice). So here’s some real ones that friends and patients experienced this week:
Real Life Example One: Sunday night you have dinner at an inside hightop table of your favorite restaurant — the one that plays 60s R&B really loud — with your beloved gal pal. She has buffalo wings that are so hot that she keeps yelling “Whooooo, baby!” and leaves her grody napkins all over the table but you love her anyway. The next day she gets tested for work and OMG she’s positive.
What does this mean?
a) It means she was contagious when you had dinner.
b) It also means she was probably contagious for a few days before that and should tell her friends and family.
c) It means she should stay home self-isolated for ten days after her test even if she doesn’t get symptoms.
d) You are at risk now to have Covid because you were inside, you were close to her, she was yelling, and maybe because of her napkins. You also need to self-isolate.
e) It does not mean you should get tested that night — you should wait at least five days because the chance of a false negative is too high as you just haven’t had time yet for the virus to multiply inside you.
f) It means next time you go out with a friend you will eat outside on their quiet patio at a big table, or just take the dang wings home.
Real Life Example Two: you start to feel achy and “off” on Monday so you stay home. On Tuesday you work from home, on Wednesday you spike a temp and have a headache. Should you have gotten tested on Monday?
Probably not; the false negative rate on Day One of symptoms is around 40% but that’s cut in half to 20% by Day Three.
What does this mean?
a) It means if you go in as soon as you get a symptom, and then the test is negative, you should not consider it an absolute negative. If you stay sick — particularly if you get sicker — you need to stay in touch with your doctors and go get another test.
b) It means you did good because you paid attention to your body and you paid attention to society and you stayed home when you didn’t feel well. Good job!
c) It means you need to continue to stay home for ten days after symptoms start as you remain contagious during that time. (Please note that this can be longer if you have severe Covid (eg hospitalized) or are immunocompromised).
What do you think will happen if you go get tested three weeks later? Will you be negative? Yes, most of the time. The infectious virus has left your body.
But what if you are positive after three weeks? Does this mean you are still contagious? No. This most likely means there are pieces of covid RNA still in your nose, but no real, live, healthy, intact, whole, and most importantly CONTAGIOUS corona viruses.
Positive tests after ten days do not seem to mean you are contagious any more
(or maybe 20 days if you had really severe Covid, were hospitalized, etc)
Sometimes we can feel so powerless in the face of this virus but the reality is we are stronger than we think. All we need to do is follow the science: mask up, keep physically distanced, stay home when sick, wash our hands, get tested when appropriate, and the transmission buck can stop with us.
(Robin Schoenthaler, MD is a Boston-based cancer doctor who has been writing straightforward fact-based no-blame-no-rumors-all-science-all-the-time essays about Covid since March 2020.)