Many people are wondering if that bad cold they had back in February or March was actually the new coronavirus. In principle, an antibody test could tell you that, but most of the tests out there right now aren’t very good. We chatted with David Kroll to learn how these tests work, where they go wrong and why a test that’s 96% accurate is actually pretty bad.
A few weeks ago I came down with a fever and started to feel really achy, kind of flowy. I’m feeling a lot better, but I still have this cough that hasn’t totally cleared up. Okay.
Did I have COVID-19?
I didn’t get tested so I don’t know, but if I did, it might mean that I’m immune, which means I could go to a yoga class, go to those three comedy shows that I had tickets to last month or go to the national Arboretum with Yuca. She misses it there. And not worry about risking my health or the health of other people.
If we could all find out if we fought off COVID-19 it could mean getting back to normal life.
Here’s where an antibody test would come in.
David Kroll: What these antibody tests are trying to do is trying to determine if your immune system has somehow reacted to the virus. They will. They will show that you were exposed to the virus long after the viral particles have disappeared long after your body has cleared the viral particles.
David Kroll: So what we’re essentially trying to do with these antibody tests is open up a history book to see whether you’ve been exposed to the virus at some time in the past.
Here’s the thing, although there are now some tests that are showing a lot of promise, it took us a while to get to this point and there are still a lot of tests out there that don’t seem to work well.
They’re like the reactions team’s animators during quarantine. They make a lot of big claims about what they can do, but then they don’t really deliver much of anything.
So what’s the holdup? Why has it been so hard to get a test? That’s good?
Let’s first look at how they’re supposed to work.
Antibody tests look for antibodies against SARS-CoV-2, usually in your blood. Antibodies are proteins that your immune system creates when you’ve been exposed to a virus or some other foreign invader and then they help your body fight off that invader if you ever see it again.
The antibodies that most SARS-CoV-2 antibody tests are trying to detect are immunoglobulin M and immunoglobulin G.
IgM is the first type of antibody your immune system makes when you’re fighting a new infection. IgG arrives later when you’ve cleared or almost cleared an infection and it sticks around a lot longer.
You start the test by adding a few drops of your blood to the test strip. Your blood starts to move down the strip where it comes across SARS-CoV-2 antigens.
Antigens are parts of the virus that your immune system might recognize like a piece of the spike protein.
If you have IgM and IgG antibodies that are specific to SARS-CoV-2 antigens, they’ll bind to those antigens through hydrogen bonding, London dispersion forces and other chemical interactions.
In the antibody test, the antigens are attached to golden nanoparticles, which will be important in a second. So the antibody antigen and that gold nanoparticle continue to move down the strip until they reached the M line.
There are actually antibodies on that line that will bind to any of the IgM in your blood sample, but you’ll only see IgM specific to SARS-CoV-2 because they have those gold nanoparticles bound to them.
As the gold nanoparticles accumulate, a line will form. A little further down the test strip the same thing happens with IgG.
There’s another antibody that we didn’t mention that binds at the control line. It shows that your blood sample made it all the way down the test. You should always get a line there.
Let’s say this test is 100% accurate. You might see a combo of things.
A line at M and C would mean that you’re maybe still fighting off SARS-CoV-2 or you fought it off more recently.
A line at G and C would mean you are at the end of that fight or maybe even fought it off quite a while ago.
off quite a while ago.
And a line at M, G and C would mean you’re somewhere in between.
Just a line at C would mean that you don’t have any antibodies specific to SARS-CoV-2 so you probably weren’t exposed to it.
It seems straightforward. Ish.
Antibody tests are not some crazy new idea, so why are there so many out there that are not reliable?
That comes down to issues with sensitivity and specificity.
Some of the tests on the market right now are still only 95% sensitive. That sounds really good, but it means that for 5% of people who do have antibodies, this test will tell them that they don’t because it’s not sensitive enough to detect low levels of antibody that is called a false negative.
So that 5% of people can’t go back to visiting friends or family or getting a haircut as soon as they actually could.
False negatives, huge bummer, but they’re not putting you or anyone else in danger.
False positives on the other hand are dangerous.
Those come from issues with test specificity. So the SARS-CoV-2 antigens and the test are not specific enough to only bind to SARS-CoV-2 antibodies.
They’re binding to other antibodies, your immune system made against entirely different pathogens.
So you go out, you meet up with friends thinking, I can’t get COVID, I already had it, but in actuality you absolutely can get sick and you can give it to other people.
Right now there are still a bunch of tests floating around that have a specificity of 96% or less.
96% might sound really good, but let’s think about what that means.
If you gave it to a hundred thousand people, 4,000 of them would get a false positive and reenter the world. That’s 4,000 people who could still get sick and spread it to others.
And since this virus is incredibly contagious, we might very quickly have a second surge of infections on our hands.
So yeah, a test with 96% specificity is actually really bad.
And to get back to normal life safely, we actually need a test that’s really, really close to a hundred percent specific and a hundred percent sensitive. Very recently there’ve been a few tests that have almost met that requirement, which is great news.
So things are headed in the right direction, but even if we got a test that was 100% specific and 100% sensitive tomorrow, there are a couple other potential problems.
First, we haven’t actually confirmed that having these antibodies will prevent you from getting sick again, that’s a huge question to tackle and deserves its own separate video.
Second problem, even if they do, we don’t know how long they’ll stick around in your bloodstream.
Antibodies against some strains of influenza can go away after just a few months. Whereas antibodies against say measles can actually stick around and protect you for decades.
So the question is where does SARS-CoV-2 fall?
In other words, could I be immune today but then no longer immune six months from now?
David Kroll: The question of how long the antibodies remain in your system is probably going to be one of the most important mysteries for us to solve in the in the coming weeks and coming months and some information that we do have from the previous SARS illness. The one that struck China and Canada in 2002 in 2003 tells us that the antibody response we generate is not very strong and not very long lived.
People who fought off the original SARS a couple decades ago showed immunity for about three years. Will that be the case with this virus? We don’t know.
So even if you are immune right now, that immunity might not last long. To make sure we’re all fully safe and protected, we need a vaccine and that’s something that people are working on.
There is still a bunch of unanswered questions and scientists are trying to answer them as quickly as possible. No, there’s no amazing test that’s just out on store shelves and you can go buy it right now.
Dr. Kroll says that we should keep an eye out for tests that had been reviewed and authorized by the FDA.
If you’re feeling frustrated, being stuck at home, we totally feel you. But I’m trying to remind myself that there are so many people out there who are working tirelessly to try and get us back out in the world as quickly and as safely as possible.