By Adam Townsend
Get online. Order a little box containing a coronavirus test kit in the mail. Swab your nose with the Q-tip inside, mail it back and wait a few days for a call or email.
Now you know whether you have COVID-19 coronavirus. And you can either self-isolate, go to the hospital, or get on with your life.
That's not possible for anyone in the U.S. as of this article's publication April 21, but short of a miracle vaccine, it's an ideal strategy for handling the COVID-19 pandemic.
And now, at-home COVID-19 screening is one step closer: The FDA Tuesday authorized LabCorp's home test collection kit for COVID-19 coronavirus infection under emergency use designation.
Mass at-home testing will not only help people know when it's safe to go back to work, but it removes a major infection threat to frontline healthcare workers manning testing stations, according to the FDA.
Neither LabCorp nor the FDA has announced details on the manufacturing volumes and timetables for these sample collection kits, so we don't know when they will be available to the public.
The home test sample kits are a step away from a fully at-home test; you have to send the self-collected samples back to the lab for analysis.
LabCorp's test works by using PCR (polymerase chain reaction) to detect viral DNA. This is the same test used to map your genome for genealogy websites or identify trace forensic evidence from crime scenes. In this case, the technology is harnessed to track down the bits of RNA that are the hallmark of the deadly SARS-CoV-2 virus that causes COVID-19.
Other tests for COVID-19 can tell you whether you were exposed at some point by detecting antibodies to COVID-19 in the body (serology tests), but none of these are approved for at-home sample collection.
To use the LabCorp kits, according to the FDA:
- You swab inside your nose according to the instructions and seal the swab in saline.
- You must use a sterile swab that comes in LabCorp's prepared sample collection kit.
- Using swabs from different manufacturers may compromise results.
- Then, you mail the samples back to LabCorp where they test them for viral DNA.
Unfortunately, PCR coronavirus tests from any manufacturer can issue false positives because you may have some viral particles in your nasal passage, but not enough to be infected and contagious.
More likely with PCR are false negatives because the virus may colonize deep in the respiratory tract. That can make it difficult to collect particles on the swab.
Dr. Hao Feng and colleagues warned fellow doctors in a Japanese Journal of Radiology article that COVID-19 PCR tests don't always return correct results, even for severely infected people.
Feng outlined a case in which a person was hospitalized for general viral pneumonia, but returned four negative COVID-19 PCR tests during the first five days of hospitalization before testing positive. This is just one of several reports in the medical literature urging caution in reading PCR test results.
How Do RT-PCR Tests for COVID-19 Work?
The RT-PCR test authorized by the FDA stands for "Real-time Polymerase Chain Reaction." It's a way of using chemicals to multiply copies of trace DNA to detect the presence of a specific organism – like taking a tiny drop of blood and tracing it to a criminal. But in this case, the killer is a tiny speck of RNA wrapped in protein.
Although tests kits have a wide variation in their procedures (swabs of oral and/or of nasal surfaces, differences in solutions to transfer the swabbed tissue to, time and place to run the subsequent test reagents, for example), they all use methods that detect a specific part of the viral genome, writes Charles Patrick Davis, MD, PhD a MedicineNet author, emergency medicine expert, and microbiologist.
Dr. Davis has been poring over all the latest literature since the pandemic began. He offered an example of how a COVID-19 RT-PCR test (generally termed a "molecular" test) works:
- Swabbed tissue is loaded into a standard reaction vial
- Reagents (the general term for additional chemicals needed in such tests) break open viruses to expose viral genome (RNA) to other reagents like synthesized RNA that binds to a specific small part of the viral RNA
- This small bound-up strand of viral genome and reagent is then replicated many times (in some tests, up to a billion times!) over minutes to hours
- Then, at the same time, another reagent specifically binds to each replicated genetic complex
- This reagent has a bounded marker that, when enough replicated complexes are made, the machines can detect the attached marker
- For example, the marker may be designed to glow with fluorescence, which the machine can pick up on its sensors
- Depending on how many viruses are present in the sample, positive results may occur (depending on the test used) in minutes to days
- Conversely, if the test detects no viral material after a fixed time (minutes to days), the result is a negative test
The qualitative results from these tests are either the person is infected with SARS-CoV-2 and considered to be able to transmit the disease (a positive test) or is negative for the virus.
This test cannot tell whether a person is immune from past infection or has yet to be exposed and is still in danger, however.
What Are Immunoglobulin Detection-Based (Serology or Antibody) Tests for COVID-19 Coronavirus?
Immunoglobulin detection tests are based on the qualitative detection of IgM and IgG that are specifically generated by the body in response to SARS-CoV-2 infection, Dr. Davis said.
No at-home serology testing or collection kits are authorized for use in the COVID-19 pandemic as of April 21, but antibody detection is the only way to tell if you had COVID-19 in the past and recovered, meaning you're likely immune.
IgM is usually the first, specific antibody type generated by the body in response to infection. Then, the IgG antibody type is generated and replaces IgM as the predominant antibody in the response to infection.
IgM and IgG fight infections by targeting specific antigens of the SARS-nCoV-2 virus.
In general, immunoglobulin tests use specific viral antigens to detect the IgM and/or IgG antibodies against those antigens, Dr. Davis said.
Here is a sample of how the test works:
- Collect 2-3 drops of fresh blood/serum or plasma and place it in a sample container and place 2-3 drops of provided buffer in the same container (cassette)
- The cassette allows the diluted sample to move through the cassette by capillary action
- The cassette has labeled SARS-CoV-2 antigen that may bind chemically with either IgM or IgG; thus, forming an antigen/antibody complexes of antigen/IgG and/or antigen/IgM
- After that reaction, the antigen/antibody product passes over anti-IgM and anti-IgG antibodies that are immobilized in a line within the cassette
- The anti-IgM and/or anti-IgG then will capture the specific complex and signal a result (a red line) if either complex is bound to the immobile anti-IgM or anti-IgG
- The results need to be read after 10 minutes and no more than 15 minutes (the test resembles a home pregnancy test in this respect.)
- A third line is a control line; it indicates that the test is working properly