With new COVID variants on the rise, the importance of coronavirus testing is as important as ever. Even without the pandemic, the principles behind the two most common COVID tests--the PCR and antigen tests--will continue to play an important role in public health and disease regulation in the future. How do these distinct tests work biochemically, and why is one test sometimes advisable over the other?
The Antigen Test
First, we’ll begin with the Rapid Antigen Test--also known as the rapid COVID test--widely popularized for its portability and ease of use.
The Rapid Antigen Test functions by placing a swab of a patient’s nostril mucus into a buffer solution, which is a substance that helps resist changes in pH level to keep a solution from becoming too acidic or too basic. The sample collected is then added to the testing wells, where it will reveal results (positive or negative presence of the virus) after around 15 minutes.
But what is actually going on, scientifically?
A Rapid Antigen Test looks for easily identifiable markers on the surface of a virus using a sample from an area in which many viruses are present (the nose for coronavirus) and then indicates the presence of these proteins. These quick tests can detect the active production of viral proteins and do not require any amplification of samples, but they have several drawbacks.
Most notably, these tests are prone to revealing false negative results and may give patients a false sense of security because they often overlook lower viral levels.
That being said, antigen tests are considered very accurate when they yield positive results, and thus are an important tool for public health officials to regulate the spread of highly infectious disease and ensure that individuals rapidly know whether or not they are affected. Additionally, this type of testing is relatively cheap and runs quickly, unlike the next type of test.
The PCR Test
The PCR COVID-19 test, which stands for "polymerase chain reaction" test, relies upon some exciting biotechnology! PCR itself is a technique used outside of vaccination by scientists to help amplify small samples of RNA into DNA, which allows the PCR test to be much more sensitive and minimize the antigen test’s issue of false positives.
Specifically, after a nostril swab is collected, it is sealed and sent to a laboratory where the genetic material is extracted from the sample. This is why PCR test results take longer than at-home rapid tests!
Next, scientists use a thermal cycler to heat and cool the genes. This helps split ("denature") the normally double-stranded DNA into single strands that can then be attached to proteins and replicated.
Typically, for polymerase chain reactions, special DNA Polymerase enzymes called Taq Polymerases help elongate (anneal and extend) existing DNA strands, essentially enlarging the information stored in our genes. More and more copies are developed, until eventually--to the point of millions of copies--a patient that is infected by COVID-19 will yield genetic material with detectable portions of the SARS-CoV-2 virus genome.
Inside the PCR machine, a chemical will create fluorescent light if the virus is present, which is detected and relayed as a positive test result. Thus, the PCR test can detect even low levels of viral count, unlike the Rapid Antigen Test. That explains why many countries require(d) PCR rather than antigen tests for travelers upon entry.
In terms of public health applications, the PCR test is often preferred by those who want to feel more confident about the status of their diagnosis at the expense of the antigen test’s efficient and accessible results.
Such is a common theme in public health, where there are benefits and tradeoffs to all policies and regulations. Indeed, disease testing and diagnosis is one of the most powerful and essential tools of public health. For that reason, innovating many kinds of vaccines for just a single illness can help more effectively mitigate the impact of infectious disease.
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