- Antigen test: These tests use lab-made antibodies to search for antigens (substances that cause the body to produce an immune response such as triggering the generation of antibodies) from the Covid virus. To run an antigen test, you first treat a sample with a liquid containing salt and soap that breaks apart cells and other particles. Then you apply this liquid to a test strip that has antibodies specific to Covid painted on it in a thin line. Just like antibodies in your body, the ones on the test strip will bind to any antigen in the sample. If the antibodies bind to coronavirus antigens, a colored line appears on the test strip indicating the presence of Covid. These tests are cheap and easy to administer at home, but they can be less accurate than PCR tests (described next). This is because unlike PCR tests, antigen tests don’t amplify the thing they are looking for. This means there needs to be enough viral antigen in the sample for the antibodies on the test strip to generate a signal. In the early stages of infection, there is not a lot of viral load in the nose, causing antigen tests to miss early cases of COVID-19. The popular Binex Now Covid tests (https://www.emed.com/airline-travel…) are an example of antigen tests
Nucleic Acid Amplification Test (NAAT): These tests involve taking a small sample of genetic material from a part of the body where the viral genetic material is likely to be found, and amplifying it (hence the name “Nucleic Acid Amplification Test”) using the natural machinery of DNA replication billions of times so it becomes possible to detect. Polymerase Chain Reaction (PCR) test is the most common example of such tests, but there are several others with fancy sounding names – Nicking Endonuclease Amplification Reaction (NEAR), Transcription Mediated Amplification (TMA), Loop-mediated Isothermal Amplification (LAMP) to name a few. Since Covid infections start from the upper respiratory tract and make their way down to the lungs, a nasal swab is a good place to start. I’ll briefly describe how RT-PCR tests work, because they are the most frequently asked for Covid-19 tests. Because viruses are balls of RNA rather than DNA, the first step is to treat the sample with an enzyme called Reverse Transcriptase (RT) that converts RNA into double-stranded DNA. This is where the “RT” in RT-PCR comes from. Then, the DNA is mixed with a solution containing an enzyme called a Polymerase and heated, causing the DNA to separate into two single-stranded DNA pieces. The temperature is lowered, and polymerase, with the help of a small piece of guide DNA called a primer, binds to the single-stranded DNA and copies it. The primers ensure that only coronavirus DNA is amplified. You’ve now created two copies of coronavirus DNA from the original piece of DNA. This is very similar to how DNA is copied during cell division. PCR lab machines repeat these heating and cooling cycles 30 to 40 times, doubling the DNA until there are a billion copies of the original piece (2^30 ~ 1B). The amplified sequence also contains a fluorescent dye that is read by a machine. The amplifying property of PCR allows the test to successfully detect even the smallest amount of coronavirus genetic material in a sample. This makes it a highly sensitive and accurate test.
Antibody tests: These tests detect the presence of antibodies produced by the body to fight Covid-19. Because it can take several days for the body to start producing antibodies, these tests can’t detect new Covid-19 infections.
It is that time of year when people are traveling and probably confused about the several options for Covid testing, so thought I’ll write a primer on this topic. I’ll also recount my experience yesterday getting a RT-PCR test done in a hurry, which others may find helpful.
First, the Covid testing primer. There are 3 main tests for Covid:
Now you’d think the U.S. would have higher standards for Covid-19 testing for entry than other countries. However the CDC accepts both antigen and NAAT tests for entry to the US.
Many other countries such as Ecuador are more discriminating and only accept RT-PCR tests. I had taken a Binex Now Covid test thinking that would be sufficient for traveling to Ecuador, however Michael pointed out yesterday that Ecuador requires RT-PCR and the Binex Now being an antigen test may not be accepted, leading to disastrous consequences for my trip. Upon realizing this, I frantically looked around for options to take a RT-PCR test and still get results in time for my flight. Thankfully, same day RT-PCR tests are available at several airports, including DCA and IAD. They cost a pretty penny (~$250), but you get the results within 30 min. So, following https://www.flyreagan.com/xpres-check, I made an appointment to get a test taken at DCA.
Upon arriving at the airport, I found that the test site was post-security, and I needed a boarding pass to go through security. I asked the attendant at the Jet Blue check-in counter if they could somehow give me a pass to go through security to take the test. The attendant said no initially, but upon some cajoling and pleading, spoke to his manager and gave me a “gate pass” that let me go through security.
After waiting for an hour to get through security, I was finally able to get to the Covid testing site. The rest of the experience was straightforward. They took my money, collected a nasal swab and asked me to come back in 30 min. The test came out negative. I breathed a sigh of relief, ensured that the test result had the word RT-PCR on it, and drove home to resume packing. Whew!