Over the past ten months, you’ve probably heard a lot about COVID-19 tests—how many are available, what the testing rate is across various regions, where you can get one in your area. You may have even had one yourself. But do you know how they really work? And what exactly does a “negative” or “positive” result mean?
Take a closer look!
The infamous nasal swab is the first step in this process. The swab resembles a 6-inch Q-tip or pipe cleaner. The purpose of this “brain tickle” is to collect a sample of the cells and secretions within the nasal passages. There tends to be a higher concentration of virus particles in this area—true of all respiratory viruses. Why does the swab have to go so far up, and wiggle around? Well, it doesn’t necessarily—some swabs collect samples from just the nasal wall, or from the back of the throat. But a really good specimen includes cells from the entire nasal passageway, so a nasopharyngeal (NP) swab is the gold standard of reliability. It might feel slightly odd or uncomfortable, but it won’t damage your brain! (In fact, it won’t even go near it).
Photo: cdc.gov
Here comes the science.
The swab is sealed, then analyzed in a lab setting to look for the genetic markers of the COVID-19 virus called RNA. This first requires removing everything that isn’t RNA—proteins, fats, etc.—from the sample with a chemical wash. The remaining RNA could be from the coronavirus, but it could also be from other viruses, or from the host (you!). So, lab technicians must then put it through a RT-PCR process. This basically reverse engineers the viral particles into DNA and compares them to a target fragment (sort of like a little test piece of COVID-19 DNA). If you have any COVID-19 particles of your own, the process will cause them to amplify them into the billions, to the point where they can be measured. Any detectable level of DNA is considered a positive result. A negative test result means that no DNA was detected.
Wait times for test results seem all over the map.
Interestingly, the time it takes to analyze any given sample is actually quite consistent. The wait time for results depends on whether the specimen is tested onsite or sent to an outside lab—and if it’s sent off, how long it takes to get there. Some labs have backlogs causing delays as well.
Testing 1, 2, 3
Most of the time, the term “COVID-19 test” refers to testing for the virus (there is also test for COVID-19 antibodies, which is a blood test that determines whether you’ve had the virus in the past). Antigen tests, aka rapid tests, work by measuring the immune system’s response to the virus. They are quicker and less expensive which makes them more practical in some cases, such as testing large groups quickly or more frequent testing. However, antigen tests are less sensitive, especially in later stages of the disease. False negatives are more common. Saliva testing was recently approved for emergency use by the FDA and shows promising signs of being comparable to nasal samples in accuracy, but it’s not widely available yet since it requires a different type of laboratory process.
What was that about false negatives?
They do occur. It could be because of a problem with the sample collection, or damage during storage or transport. It can also happen if the test subject isn’t shedding enough of the virus when the test was taken (you may have heard of people who tested negative one day and positive the next). False positives can also occur during testing when the target fragment replicates a virus that is similar to COVID-19, but is not the virus itself. That’s why standardized test kits are so important. The type of test you take could make a difference too.
Accurate testing is essential for monitoring and controlling the pandemic. Gracelight Community Health are proud to perform COVID-19 testing at all of our locations. Visit the Gracelight Community Health COVID-19 testing page or call (323) 635-1900 for more information.