COVID19 Testing

Introduction

In the midst of the current COVID19 pandemic testing has been a major topic with stories on the news every day.  Rapid progress is being made in the diagnosis and treatment of COVID19 but because of this rapidly changing information it can be difficult to understand what tests are available and what the differences are. This page was created to help summarize the basics of the currently available or "soon to be available" tests

Types of Tests

Simply put there are two main types of COVID19 tests. There are tests which are designed to detect a CURRENT INFECTION and tests designed to look for evidence of PAST INFECTION.

CURRENT INFECTION

The "SWAB TEST" - Commonly referred to by patients as the swab test, these tests are done by using a swab to sample fluid and cells from the nasal cavity. The objective is to obtain cells that contain the SARS CoV-2 virus that causes the COVID19 disease.  SARS CoV-2 is an RNA virus which means that unlike living cells which store their genetic information in the form of double stranded DNA, this virus stores its genetic information in the form of a slightly different molecule called single stranded RNA.
When we develop a test to look for the presence of the virus there are two ways to do this. One is to look for the specific type of RNA made by this virus and the other way would be to detect the proteins on the surface that are specific to that virus. Both of these tests are done by collecting samples from the nasal passages or sometimes the throat or saliva. There are important differences between these two types of tests.
  • PCR TEST - This is the main test that you have been hearing about and the one that looks for the RNA.  It's the primary test that people have been getting done since the beginning of the COVID19 outbreak. There are many companies producing variations of this test but they are all PCR tests. PCR (Polymerase Chain Reaction) is a technique that allows us to look detect even the tiniest amounts of viral RNA. We all have lots of RNA in our bodies. Our cells use their own RNA to make proteins. So to perform this test the lab has to identify not just the presence of RNA but it has to be able to distinguish between human RNA and COVID19 RNA.  When the PCR test is performed only a very tiny amount of the viral RNA is present on the specimen collection swab. The amount is so small that the viral RNA would normally be impossible to detect so technicians need to amplify the number of copies of the genetic material that was collected. This is what PCR is used for. PCR is like putting the RNA through a copy machine. Using this technique they can make billions or even trillions of copies of the viral RNA from a  single strand of the original RNA . Once this is done there is enough viral RNA that the lab can then test the RNA sequence to see if it matches the virus.

    Like any test there are advantages and disadvantages to this test.
    • Pros  
      • Extremely sensitive test
      • Easier to develop. Therefor its the first test available when a new virus evolves
    • Cons 
      • Does not distinguish between viable virus and genetic remnants left over from destroyed virus. As a result a positive test does not necessarily mean that a patient has an active infection or that they are contagious since the test may be picking up genetic material left over from an infection that resolved days ago.
      • Sometimes too sensitive - Can detect tiny amounts of genetic material. May be detecting small amounts of virus that contaminated the swab, the testing equipment, or the patient but aren't currently causing infection.
      • Requires a sample from deep in the nasal passages. This tends to cause coughing or sneezing. For this reason the healthcare personnel doing the test must wear full PPE and must perform the test at sufficient distance from other workers and patients. These requirements  can limit the availability of the test.  
      • Failure to get a sample from deep enough in the nasal passages can lead to false negatives
      • This test is more complicated to perform than other tests . Therefor it takes more time to do properly and is more difficult to scale up to very large numbers

  • ANTIGEN TEST - Not to be confused with the Antibody test (see below).  An AntiGEN is a protein on the surface of the virus or bacteria where as an AntiBODY is a protein produced by our immune cells which attaches to that viral or bacterial antigen. The antigen test is designed to detect one of the unique proteins (antigens) on the surface of the virus. In this case the spike protein (see above) is the antigen that the test looks for. Again this test is done with a sample collected by a swab in the nose or throat.
    • Pros 
      • Less complicated test
      • Fast results
      • Easier to manufacture and scale up.
      • Usually less expensive
    • Cons
      • May not be as sensitive as PCR
      • More difficult to develop so may not be available at the early stages of a pandemic when a new virus first arises. There is no FDA approved antigen test as of this writing. The first such test was only given an emergency use authorization (EUA) by the FDA on 5/10/20. This means the company says they have validated the reliability of the test but the FDA has not had a chance to confirm that yet. The test may not be publicly available for a few weeks yet.

NY Testing Locations

COVID-19 testing is available through the state. You can schedule an appointment through the link or phone number below

WebSite 
covid19screening.health.ny.gov
Phone
888-364-3065

NOTE: This information is for the nasal swab testing. Nasal swab testing is for people who may be currently infected.

Antibody testing is being offered in some places but is not recommended at this time due to problems with the test that can give misleading results.

PAST INFECTION

ANTIBODY TEST - Commonly called the ANTIBODY TEST, this test requires a blood sample. Tests that look for past infection are not looking for the virus itself like the PCR and Antigen tests discussed above. These tests are looking for the antibody that your immune system generates to attack the virus. Our immune system is incredibly complex in its response to infections but one portion of the immune system known as the adaptive immune system fights infections in part by creating antibodies that attach to unique proteins on the surface of a virus or bacteria. Antibodies are small molecules that the immune system creates that are an exact match for the shape of a specific viral or bacterial protein. They fit together like a lock and key. When the antibody attaches to the protein it signals other immune cells to come over and destroy the thing it is attached to. The first time you get an infection it takes a week or two for the immune system to learn how to make that special antibody but after the infection is cleared there are memory cells that retain that ability and can be called upon if the same organism tries to cause infection again. This is why we are immune to many bacteria and viruses if we have been infected by them once before or if we have been vaccinated.

The ANTIBODY TEST is a test that is designed to look for the antibody that our bodies would have produced to attack whatever virus we are interested in. It's important to note that not every antibody provides immunity. Some antibodies attack a portion of the virus or bacteria but are not able to trigger the necessary immune response required to destroy it. The COVID19 virus triggers the formation of several different antibodies by the immune system and we don't know yet which, if any, will be protective.  Antibody tests are done with a blood sample not a swab. As  of 5/10/20 no Antibody test has yet been FDA approved. Some facilities are claiming that they have been approved but they have only been given an Emergency Use Authorization (EUA) which means the company says the test works but the FDA has not verified that yet.

Just like the swab tests this test has some pros and cons.

  • ANTIBODY TEST
    • Pros
      • Can detect whether someone has had infection even after the infection has resolved
      • Can be used to estimate what percent of a population has already been infected
      • May signal immunity to an infection
      • Simple to perform
      • Can be scaled up and done in large numbers
    • Cons
      • Can not tell if someone is currently infected
      • Requires more study to determine if the presence of the antibody provides immunity to the disease. This can only be determined with certainty when people who are known to have a positive antibody are then re-exposed to the virus.
      • Multiple antibodies are produced with COVID19. It needs to be determined which antibody is protective and if all of the available tests are testing for the same antibody.
      • Some of the existing tests are giving false positive results because they cross react with a protein found on the surface of the Coronavirus that causes the common cold.

As of the date of this article (5/10/20) an Antibody test does not yet provide any useful information since the results may not be reliable and a positive test can not be interpreted as providing protection yet.

Michael Melgar, MD