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doctormike · 4 years

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Testing for COVID-19

Dear families and friends,

I have been getting a lot of questions from my patients' parents about testing for COVID-19. I know that many of you don’t have the time for this whole thing, so here’s the TLDR version:

None of the tests have formal approval by the US Food and Drug Administration, but many have been given Emergency Use Authorization (EUA) pending approval.

There are two types of tests, diagnostic tests that tell you if you currently have COVID-19, and antibody tests which tell you if have had it in the past.

In my practice, I mainly use diagnostic testing for patients having surgery (all facilities currently require this within 72 hours of the operation).

Patients being tested because they are sick and COVID-19 is suspected are better referred to open facilities with lots of protective equipment, rather than in individual doctors offices.

There really is no urgent medical need to do antibody testing in the office right now, although many people request it out of curiosity. At this time, it is mainly indicated for large studies monitoring exposure rates in a population or for people who have recovered and want to donate plasma to treat others.

A positive antibody test does NOT necessarily mean that you are immune, or that you can’t transmit the disease even if you are. This is a new disease, and we are still learning about it. Everyone should continue to follow local recommendations about hand washing, mask use and social distancing.

Things are changing rapidly, and this email may not age well. So keep up to date on this topic as it develops.

PLEASE make sure that your kids’ regular vaccines are up to date - this can be done safely in your pediatrician’s office. This pandemic should make it clear just how dangerous viral infections can be.

OK, if you are still reading, let’s get into the details. While actually getting a test is becoming easier over time, there still seems to be a lot of confusion about the nature of the tests, and what they do or do not demonstrate. Testing would be best done through your pediatrician's office, although I have some access to these tests in my office as well. But just because it is possible to get one, that doesn’t mean it’s always a good idea.

Before we start talking about tests, let me just clarify some terms:

Coronavirus: A large family of viruses, which includes the one causing the current pandemic, as well as SARS (2002), MERS (2012) and some of the common cold viruses

COVID-19: An abbreviation for “Coronavirus Disease 2019". Technically, this refers to the disease, not the virus itself.

SARS-Cov-2: The virus that causes COVID-19

It’s helpful to know this when thinking about testing, even though most people just use the term “COVID-19” and everyone knows what they are talking about. I will use that for clarity in the rest of this email, even though I may be talking about the virus and not the disease. Also, there are a lot of details about the mechanics of testing that I have simplified here, since I didn't want to make this more complicated than necessary. Finally, remember that you are seeing medical progress happening MUCH faster than ever before, so this email may not age well. Keep watching trusted sources of information for new developments and tests as they happen.

You should understand that while there are no FDA-approved tests for COVID-19, there are a VERY large number of tests from different manufacturers, and some have been granted EUAs (Emergency Use Authorizations), since formal FDA approval takes so long. This is because this is a totally new virus, and because of the urgency of the situation, especially in the first few months of the pandemic. But many of these tests are not very accurate, and the FDA is now starting to revoke authorizations for companies that can’t show that their tests are good enough. In many ways, inaccurate testing is worse than no testing, since it can lead doctors and patients to make bad decisions. The FDA is trying to limit its EUAs to manufacturers that can meet certain standards - here is their current list of these tests.

So let’s talk about the different types of COVID-19 tests. The two main types are diagnostic tests (which tell you whether you are currently infected with the virus), and antibody tests (which tell you if you have been infected with the virus in the past). They are both important in the fight against this pandemic, but they have very different roles.

Most of the diagnostic testing so far has been by molecular tests, which detect genes that are unique to the SARS-Cov-2 virus. This is usually done by a chemical reaction called PCR, which makes huge numbers of copies of the virus’ genetic material, to the point that the small amount in a swab of the nose can be detected in a lab. The PCR test has mainly been done so far by nasopharyngeal swab, which is a thin culture stick placed in the back of the nose, and takes a few days to come back. It seems that a less invasive test based on saliva will soon be widely available as well, and some rapid and even home-based molecular tests that can give results in less than an hour – watch for these developments. In addition to determining whether or not a patient actually has COVID-19 at the start of an illness, a diagnostic test can monitor for how a patient clears the virus. This information can help doctors make decisions about hospital discharge, and whether or not a patient is still contagious. The CDC points out that you can make that call either by testing (two negative diagnostic tests a day apart), or by time (3 days after recovery or 10 days after the onset of symptoms).

Another type of diagnostic testing looks for proteins on the surface of the virus (antigens) instead of genetic material. The first of these antigen tests got an EUA from the FDA on May 8; these are cheaper, faster, and can be done at a scale appropriate for widespread testing. If this test comes back positive, it is very accurate, but it is much more likely to miss an infection than the molecular tests. So if the test is negative and a doctor still thinks that a patient might have COVID, then they would need PCR testing. Still, this could be useful for screening large populations if done regularly. People testing positive would be identified much more quickly and quarantined, which would slow the spread of infection. People who had the virus but were missed by an antigen test could get picked up on the next round of testing, since this could be done much more frequently. While of course it would be better to not miss any people with the virus, that doesn’t mean that the antigen test is worthless, especially if it is done widely and often in high-risk populations. Since COVID-19 patients shed virus early in the course of an infection, the sooner you can identify and isolate an active case, the better.

Now let’s talk about antibody testing (also called serology testing). When a patient is exposed to anything that isn’t a part of their body (a virus, bacteria, or even toxic chemical molecules), the immune system defends them in a number of ways. One way is the production of proteins called antibodies, which stick to a virus. Specifically, they stick to antigen proteins on the surface of the virus. This helps the immune system control the attack by blocking the virus’ ability to infect cells, by triggering the production of proteins that can destroy the virus, or by making the virus or infected cells easier to consume by other cells in the immune system.

There are several types of antibodies, but the ones that we test for in COVID-19 are IgM (which is the first one to appear), and IgG, which can last a long time after infection. Vaccination is a way of getting the body to produce IgG so that the next time the virus shows up, the immune system is ready for it. While there is no vaccine for COVID-19 yet, all of the vaccines that children get work in this way – they use a weakened or dead form of a virus or bacteria to get the immune system to produce antibodies.

This brings up a VERY important point. I know that a lot of you are worried about going out of the house, even for doctor's visits. Let me just tell you again that it is EXTREMELY important for your children to get their regular vaccines! Some of these diseases are far more dangerous to your children than COVID, and your pediatricians will be doing all that they can to make office visits safe. PLEASE do not skip these, especially for younger children.

Now that we know what the different types of tests are, let’s talk about how good they are. For any medical test, it’s important to know two things – sensitivity and specificity. The tests that we are talking about detect either a disease (COVID-19) or some other condition (the presence of antibodies). If they are positive, that means that the patient has the infection, or has antibodies. Sensitivity is how likely the test is to pick up the condition; specificity is how likely the test is to be negative if the condition is NOT present. Both of these are measured as a percentage. A test with good sensitivity will have very few false negatives (you test negative but you actually have that condition). A test with good specificity will have very few false positives (you test positive but you DON’T actually have the condition). This is pretty complex, and for situations where most people don’t have the disease, unless the specificity is extremely high you are going to have a lot of healthy people testing positive. if you are interested in comparing the accuracy of different tests out there and seeing what happens with different clinical situations, here’s an interesting simulator to play around with. If you want to see how the simulator works with the real data on the 13 antibody tests for which the FDA has granted EUAs, that information is here.

So what do I do with these tests? In my ENT practice, I am mainly using the PCR test for patients who are not sick but who are having elective surgery. Operating rooms are reopening, but they all require a negative diagnostic test within 48 or 72 hours of the operation. These swabs are still in limited supply, so we are trying not to use them in other situations, although occasionally that might be requested (for example, determining the need for quarantine in a patient with no symptoms but who has been exposed to the virus). The other reasons for doing diagnostic testing are for patients suspected of having COVID-19, or for health care workers with minimal symptoms who work closely with others at high risk. Patients suspected of having this infection are sometimes not tested at all, especially if the symptoms are typical and mild, and they are just going to recover at home in isolation. If we decide that a test is needed for a sick patient (for example, if we are considering hospitalization), it is better to do that in an open space, in a facility with more protective precautions, so we would try not do that in our office.

What about antibody tests – should you or your child get one? Since these tests are much more easily obtained, lots of people have been getting them. Often this is done out of curiosity - someone has had an illness earlier this year and recovered at home, and they want to know if it was COVID-19. Remember, these won't be positive for several weeks after an infection, some people make more antibodies than others, and some don't make any at all. Other people have the idea that if they are positive, then they have an “immunity passport” – they don’t need to follow state guidelines for social distancing, masks or hand hygiene. There are serious problems with this line of thought.

First of all, there are a lot of antibody tests out there, and they don’t all have great sensitivity and specificity. In some facilities, positive tests are sent to a clinical laboratory where follow-up testing can be done to make sure that the positive result is actually correct. One of the reasons for this is cross-reactivity, in which the test may be positive if you have antibodies to other coronaviruses (such as the common cold). You might want to check out this page from Johns Hopkins, which has a long list of antibody tests (those with EUAs from the FDA and others as well) and some good details about how the different types work.

But even if an antibody test is very accurate, remember that all it shows is the presence of antibodies. Because this is a new disease, we still don’t know what that means. There is not yet clear evidence that people with antibodies are protected from getting sick again, or that they can’t spread the infection to others. And if there is some protection, it’s not clear how long that will last. While COVID-19 is new, you can look at other infections to see that it’s not such a simple question. For example, HIV infection produces antibodies, but they don’t protect against disease. In dengue, the antibodies actually make reinfection MORE lethal. Some diseases like measles produce antibodies that give you lifelong protection, but others (like the coronavirus SARS) produce antibodies for a while but are mostly gone after a few years. And of course, we don’t know if the SARS-Cov-2 virus will mutate, like influenza, in which changes in the surface proteins make antibodies from infection or a vaccine less effective against new strains over time. The current recommendation for antibody testing (from the AMA and other organizations) is that it not be used to make medical decisions about individual patients. We definitely do NOT want people using these test results to stop social distancing, hand hygiene or the use of facemasks in situations where these things would otherwise be needed. Antibody testing is recommended for two main things – big studies that look at how common COVID-19 is in a large population, and testing people who have recovered and want to donate blood for the preparation of convalescent plasma (a blood product that may be helpful to some very sick patients). This is a really complex question, and the details are beyond what I want to include in this email. But here is a good article that explains the issues with antibody testing in greater detail, if you want to dig in for a deeper understanding of the questions involved.

OK, that was a lot longer than I had intended! I hope that it was helpful. Just remember to watch the state guidelines on distancing and masking, and always practice good hand hygiene (even if you test positive for antibodies). Remember to vaccinate your kids against all the other disease that we can now control. And finally, if you are getting a test, make sure you understand how that information is going to change what you are going to do.

Stay safe!

Mike Rothschild, MD

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