To Test or Not to Test...That is NOT the Question!!

April 26, 2020 David Goodman

When it comes to COVID-19, here are the questions that you should be asking:

What test should I get?
When should I get tested?
Why should I believe the result?

First of all, you need to be aware that every test on the market today for COVID-19 has been approved through a special mechanism called an emergency use authorization (EUA). This provision of the US Food and Drug Administration (FDA) law allows the agency to fast track diagnostics, drugs or other regulated medical products in the event of a public health emergency such as the COVID-19 pandemic. This means that the companies who are marketing their products under EUA applications may not have not performed the rigorous testing that is usually required for submission for FDA approval. This has potentially important implications on the accuracy and repeatability of testing.

What test should I get?

The world of COVID-19 testing is divided into two distinct camps:

Molecular tests: these are laboratory tests that are performed on swabs from the nasopharynx ( a long flexible Q-tip inserted waaaay up the nostril) or from blood or other bodily fluids. These tests are all based on the detection of genetic material from the virus. By definition, a positive test means that you have the virus in your body. It does not necessarily mean that you are sick, but it does mean that you are infectious to others. As we have heard, people who are completely asymptomatic can have the virus and give it to others, like mom, dad, grandma, etc.
Antibody tests: these are laboratory tests that are performed on whole blood or serum (serum is the straw-colored liquid that remains when blood is put through a centrifuge and the red and white blood cells are separated out). These tests are designed to detect the presence of antibodies, which are protein chemicals produced by the body as part of our immune system defending against a foreign invader like a virus or bacteria. In the case of coronavirus, patients who recover from COVID-19 as well as asymptomatic carriers of the virus produce antibodies over time. By definition, the presence of antibodies means that you have been exposed to the virus.

When should I get tested?

If you think you might be infected, especially if you are having symptoms or have been potentially exposed to someone who has novel coronavirus, don’t get an antibody test, get a molecular test.

There is wide variation in how molecular testing is implemented across the US and even across counties, partly based on priority due to limited availability of tests and resources, I pulled the following off the CDC website on 4/25/20:

PRIORITY 1 • Hospitalized patients • Symptomatic healthcare workers

PRIORITY 2 • Patients in long-term care facilities with symptoms • Patients 65 years of age and older with symptoms • Patients with underlying conditions with symptoms • First responders with symptoms

PRIORITY 3 • Critical infrastructure workers with symptoms • Individuals who do not meet any of the above categories with symptoms • Health care workers and first responders • Individuals with mild symptoms in communities experiencing high COVID-19 hospitalizations

NON-PRIORITY • Individuals without symptoms

For molecular testing, the availability of at home sample collection should improve the situation dramatically because currently, obtaining a nasal swab requires going to a special location and that the healthcare worker obtaining the specimen be in full protective gear, including N95 mask, gloves, gown, face shield, etc.

For antibody testing, its main value is to identify people who may have previously been infected and have developed an immune response, as measured by the presence of antibodies to the novel coronavirus. As I said above, it cannot tell you that you are not infectious now, as only a molecular test can determine whether you have the virus present in your body. Also, the longer the period of time following infection, either known or silent, the more likely an antibody test will be positive. Antibodies typically remain detectable for months or even years. Therefore, antibody tests are likely to be more reliable if you are tested several weeks after infection.

THIS IS IMPORTANT! At this point in time, there is NO evidence at this time that people who have antibodies are immune to re-infection. You may have heard about countries like Germany issuing ‘immunity passports’ to people with positive antibody tests. We don’t know if that strategy makes sense yet. Antibody testing only tells you that you have been exposed to the virus in the past. You should not assume that you can’t get it again so don’t get rid of the mask and social distancing, etc.

Why should I believe the result?

Good question. At this point in time, I am more inclined to believe the results of molecular and antibody tests performed in laboratory settings where complex biochemical testing is routinely performed. In the rush to rapidly expand the availability of testing, there have been some notable hiccups. For example, the new Abbott five-minute molecular COVID-19 test had an unacceptably high percentage of false negative results (test result is negative even though the person actually has the virus) when it was first introduced. This has apparently been fixed by changing how the swab is handled, improving the reliability of the testing but potentially placing the people performing the test at increased risk of becoming infected through the specimen handling and sample preparation procedures.

The point of care (POC) antibody tests being sold for use in clinics and doctor’s offices work similarly to pregnancy or strep tests, that is to say a visual indicator on a cassette after a drop of blood or serum is applied. There are multiple tests that have recently entered the market with varying degrees of accuracy and reproducibility. Remember, they are all approved under the EUA process and have not been subjected to rigorous testing. The New York Times had an excellent article articulating the problem. I would not trust any of these antibody tests until there is published validation data.

Of note, the national laboratory testing company Quest Diagnostics is now offering antibody testing. It is a standard blood draw so most Quest locations can obtain the sample. I expect that LabCorp, Bioreference and the other major lab companies will be offering antibody testing as well. In contrast to the drop of blood tests that have exploded onto the market, I would believe the result from one of these lab companies as they routinely perform similar tests and have rigorous quality control procedures in place.

Putting it all together

The availability of both molecular and antibody testing is rapidly increasing. For more information, I have attached a few links.