COVID19 Testing: What you need to know

May 12, 2020

It would be really nice to know if that odd symptom we have is really COVID19 (because it seems to cause more and more things). It would be great to know if we're immune to this coronavirus that is taking the world by storm because we were sick and didn't get a diagnosis at the time. If immune, we wouldn't need to worry about catching it or spreading it. We could even be plasma donors to help others fight off an active infection.

Unfortunately, the testing that is available now isn't all we want it to be.

As with everything related to COVID19, look at the date on which it is written or updated. Things are rapidly changing!

First, a little basic virology & immunology

Virology

COVID-19 (or SARS-CoV-2) is a virus in the coronavirus family.

Other coronaviruses that are well known include SARS and MERS, but people around the world commonly get infected with human coronaviruses 229E, NL63, OC43, and HKU1.

This is important because many of us have had at least one coronavirus infection at some point. Most of these coronaviruses cause mild to moderate upper respiratory tract illnesses, what we call the common cold.

Coronaviruses are spread through the air (especially from coughing or sneezing), from surfaces (this is why we should not touch our eyes/nose/mouth with unclean hands), and general close contact with someone who is infected.

Immunology: IgM and IgG

Our immune system uses antibodies to fight off infections from viruses and bacteria. Antibodies are a type of immunoglobulin. Antibody tests look for these in blood samples.

Two immunoglobulins that come into action during infections or after vaccines are IgM and IgG. There are others, but these are the two types important in COVID19 antibody testing.

An antibody that develops early in an infection is an IgM antibody directed at the specific infecting virus or bacteria. It does not last long, but is an early responder.

IgG doesn't show up until later in an infection, but stays around longer. It is what is considered long term immunity, but the duration it stays varies. It is specific to the bacteria or virus that triggers it.

Immunizations can also trigger IgG production, which provides immunity to that specific vaccine.

Types of COVID19 Testing

There are many tests available for COVID19, but the testing reliability and best use vary.

When you're sick or exposed: respiratory tract testing

PCR: the gold standard

When someone is actively sick or exposed to someone with COVID19 disease, a nasopharyngeal swab or saliva test may be used. The sample collected is used for PCR (polymerase chain reaction) testing.

PCR testing looks for the genetic material of the coronavirus. PCR testing amplifies the viral genetic material if it is present to help identify it. If this genetic material is found, it is considered an active infection.

Tests rely on good samples and proper collection technique. One of the concerns with nasopharyngeal testing is that if the swab doesn't reach the back of the nose in the throat area, it is not reliable.

We've all heard stories by now of people with classic symptoms who had one or more negative tests before they finally had a positive test. False negative test results can lead people to think they are not contagious when they really are, which is one of the many reasons we all should presume we could be contagious and use proper precautions always.

These tests are available in limited quantities, so unless you meet local criteria for testing, you may not be able to get this test at this time.

Antigen testing

A newer type of test coming to market is an antigen test. This test looks for virus proteins in nasal samples.

Antigen tests are very specific for the virus but are not as sensitive as PCR tests. This means that positive results from antigen tests are highly accurate, but there is a higher chance of false negatives. Negative results from an antigen test may need to be confirmed with a PCR test if there is high suspicion of an active infection or high risk of spread to others.

The benefits of antigen testing are the lower cost compared to PCR testing and the results are more rapidly available.

When you suspect you were sick before: blood testing

Antibody testing can be done on blood samples to tell if you've had a coronavirus infection in the past.

Unfortunately there is concern that a previous "common cold" coronavirus infection can cross react with testing, which means you might *not* have had COVID19 if your COVID19 test is positive. This is a false positive.

It is also possible if you have had a recent (or current) COVID19 infection that the IgG antibodies are not yet present, so you could have a false negative test. These tests are not good at picking up an active infection. It can take 1-3 weeks for the IgG to show up in your blood.

Antibody testing results

Ideally a test will either be positive (you have antibodies) or negative (you don't have antibodies) but unfortunately there is no perfect test. There are false positives and false negatives. Each has its own problems.

If you had COVID19 and the test is positive, you have a true positive.

If the test is negative and you never had the infection, it is a true negative.

Some people who had COVID haven't had time to develop the antibodies yet (or maybe never will). These antibody tests will be negative despite a COVID19 infection. This is a false negative but may mean that you aren't immune even if you had the infection.

Some people who didn't have COVID19 but have had a common cold coronavirus may have a positive antibody test. This is a false positive test. This is especially dangerous because they may have a false sense of security against catching and spreading COVID19, but they are still at risk.

Many of the claims of sensitivity and specificity make it sound like these tests are very reliable, but it's all in the numbers. One example:

From "Antibody tests may hold clues to COVID-19 exposure, immunity—but it's complicated"

Those impressive-looking numbers, though still leave the possibility of a significant number of people having false-positive and false-negative tests. For example, if 5% of the US population actually had the virus, a test with 95% sensitivity and 95% specificity conducted in a million people would correctly detect 47,500 cases, along with 2,500 false-negatives (those who were infected but were missed).

But it would also produce 47,500 false-positives. So, if only 5% of the population was infected, the number of true-positives and false-positives would be the same.

"Imagine going to a healthcare worker and saying 'we're going to test you for antibodies right now, and if you're positive, you have a 1 in 2 chance it's not real,'" says Michael Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy (publisher of CIDRAP News). "Are you actually going to use that test in a meaningful way?"

What is antibody testing good for?

If you had a classic COVID19 infection, especially if you had positive testing when you were sick, an antibody test can be used to determine if your blood can help others. If you are positive, you can be a plasma donor and possibly save lives with your donation!

Even though individual test results are not always reliable, antibody test results may help us estimate community rates of COVID19 infections. This will take widespread testing, which is not yet available.

How long is the IgG protection going to last?

Good question.

No one knows.

This is why even if you had a positive nasopharyngeal swab when you were sick and a positive IgG test, which shows you most likely really did have an infection and you have antibodies now, you should still use precautions against another infection.

We don't know how long those antibodies will last. If they fade, you could be at risk again.

Be safe

The safest answer for now is to maintain precautions whether or not you've already had COVID19.

Wash hands or use sanitizer often.
Don't touch your eyes, nose or mouth unless your hands are clean.
Wear a mask when in public areas. Keep your nose and mouth covered.
Try to maintain a 6-10 foot distance whenever possible.
Stay home when sick.
Clean surfaces often.

Update:

This infographic was shared with me 5/19/2020 and it is a great visual on just how unreliable testing can be. A positive result is usually correct, but there are some false positives.

More concerning to me is the false negative rate. In this graphic, the yellow group have COVID19 but their test shows they do not. These people are at risk of sharing the virus because they relax their precautions.

Everyone should try to keep physically distant as much as possible, and wear a mask when they're in groups outside their home (and in the home if there is a risk there). Wash hands or sanitize often - especially after touching often used surfaces. Don't touch your face or your mask unless you clean your hands before and after. Be safe!