What's the Difference Between PCR and Antigen COVID-19 Tests?

At this point in the pandemic,chances are you or someone you know has been tested for COVID-19 at least once.But do you know which test you took and the pros and cons of these different tests?I'm a molecular biologist and since April 2020,I've been part of the team on an NIH-funded project called RADx,which is helping innovators develop rapid tests to detect how infected with SARS-CoV-2, the virus that causes COVID-19.Two main types of tests are used to diagnose SARS-CoV-2 infection: molecular tests (better known as PCR tests) and antigen tests.Each detects a different part of the virus and how it works affects the speed and relative accuracy of the test. So what is the difference between these types of tests?,search for genetic evidence.The first step in any kind of test is to obtain a sample from the patient.This can be a nasal swab or a bit of saliva.For PCR testing,the next step is to amplify the genetic material so that even small amounts of coronavirus genes in patient samples can be detected.This is done using a technique called polymerase chain reaction.A healthcare worker removes the sample and treats it with an enzyme that converts the RNA into double-stranded DNA. The DNA is then mixed with a solution containing a polymerase and heated to separate the DNA into two single-stranded DNA fragments.The temperature is lowered, and the polymerase,with the help of a short piece of guide DNA called a primer,binds to the single-stranded DNA and copies it.Primers ensure that only coronavirus DNA is amplified.You have now created two copies of the coronavirus DNA from the original piece of RNA.The lab machine repeats these heating and cooling cycles 30 to 40 times,doubling the DNA until there are 1 billion copies of the original fragment.Amplified sequences contain fluorescent dyes and are machine readable.

The amplification properties of PCR allow 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.It is nearly 100% accurate and is the gold standard for diagnosing SARS-CoV-2.However,PCR testing also has some weaknesses.They require skilled lab technicians and special equipment to run them, and the amplification process can take an hour or more from start to finish.Often only large,centralized testing facilities such as hospital laboratories can perform multiple PCR tests at once.Between sample collection, shipping,amplification,testing,and reporting,it can take anywhere from 12 hours to five days for a person to get a result.Finally,they're not cheap, $100 or more per test.

Antigen test 

Rapid, accurate testing is critical to contain highly contagious viruses like SARS-CoV-2.PCR tests are accurate but can take a long time to produce results.Antigen tests,the other major type of coronavirus test,are much faster but less accurate.Antigens are substances that cause the body to mount an immune response they trigger the production of antibodies.These tests use lab-made antibodies to look for antigens of the SARS-CoV-2 virus.To perform an antigen test,you first treat the sample with a liquid containing salt and soap to break up cells and other particles.This fluid is then spread on a test strip that has antibodies against SARS-CoV-2 coated in thin lines.Just like the antibodies in your body,the antibodies on the dipstick will bind to any antigens in the sample.If the antibody binds to the coronavirus antigen,a colored line will appear on the test strip,indicating the presence of SARS-CoV-2.Antigen testing has many advantages.First,they are so easy to use that even people without special training can perform them and interpret the results even at home.They also provide quick results,usually in less than 15 minutes.Another benefit is that these tests are relatively inexpensive, around $10-15 per test.Antigen tests do have some drawbacks. Depending on the circumstances, they may not be as accurate as PCR tests.Antigen tests are very accurate when a person has symptoms or has a lot of virus in their body.However, unlike molecular PCR tests, antigen tests do not amplify what they are looking for.That means there needs to be enough viral antigen in the sample for the antibodies on the strip to produce a signal. When a person is in the early stages of infection, there isn't much virus in the nose and throat,which is where samples are taken.Therefore,antigen testing may miss early cases of COVID-19.It is also at this stage that a person does not have any symptoms,so it is more likely that they will not know they are infected.more tests,more knowledgeSome antigen tests are already available over the counter, and on October 4, 2021, the U.S. Food and Drug Administration granted emergency use authorization for another home antigen test.The U.S. government is also working to make these tests more accessible to the public.In RADx, a project I am involved in, we are currently conducting clinical studies to better understand how antigen testing works at various stages of infection.The more scientists know about how accuracy changes over time,the more effectively these tests can be used.Understanding the advantages and limitations of PCR and antigen testing,and when to use them, can help control the COVID-19 pandemic.So the next time you get tested for COVID-19,choose the test that's right for you.