We’re battling the COVID-19 pandemic’s next wave of cases and new variants. The same safety measures we started with remain as important as ever. But when it comes to diagnostic tests, if we compare the PCR vs. the antigen test, which is better? Which is more accurate or more practical?
Here is a quick comparison.
The Rapid Antigen Detection Test (RADT)
This rapid antigen test is a quick way of finding active COVID-19 cases. It’s similar to the method used by doctors to test for strep throat infections.
However, this test cannot give a definitive diagnosis of COVID-19. Instead, the test is a way of initially screening people and identifying who among them needs a more thorough follow-up diagnosis. And here’s why.
How does the antigen test work?
These tests are immunoassays. A sample of secretions from deep within your nose (i.e., nasal swab) is collected. Then it is tested for the presence of antigens specific to the SARS-CoV-2 virus (which causes COVID-19).
Antigens are nucleocapsid proteins on the surface of the coronavirus. These are what trigger an immunological response in our bodies.
Note: While antigen tests are often sold as point of care (POCT) or onsite tests, Australia’s Therapeutic Goods Administration (TGA) currently does not allow these to be used as a ‘self-test.’ A health practitioner must supervise the use of these devices.
How fast is the antigen test?
These immunoassays have a rapid turnaround time. Nasal swabbing can take anywhere from 5 to 10 minutes (or even less), depending on the patient. Test results are available in just a few more minutes. The estimated total time devoted to each patient is anywhere from 15 to 30 minutes.
How reliable is it?
The principal problem with the RADT or antigen test is accuracy. According to initial research, the test provides a sensitivity that can vary anywhere from 48.8% and 76.8%.
But a 2020 study by the U.S. Centers for Disease Control and Prevention (CDC) found the antigen test’s range of sensitivity to be even lower:
• 64% for people exhibiting symptoms
• 36% for people who are showing no symptoms
This is because antigen tests need thousands of coronavirus particles present in every microlitre of sampled mucus to yield a positive result. This poses a problem when the test is performed too early, i.e., within the first 5 days of infection. During that time, many patients are still asymptomatic (but already infectious to a certain degree). As yet, they do not have a high viral load (and therefore antigens) in their system. This can often lead to a false-negative result.
The antigen test is useful only if someone has high concentrations of the virus. It tends to miss infections with low viral loads. It is therefore insufficient for diagnosing or ruling out a COVID-19 infection conclusively.
The RT-PCR Test
The RT-PCR test is the most widely-known type of Nucleic Acid Amplification Test or NAAT (sometimes also called “molecular tests”). A NAAT or molecular test is the global “gold standard” for COVID-19 diagnostic tests. Doctors use these tests to make a conclusive diagnosis of a patient.
How does the RT-PCR test work?
“RT-PCR” stands for “reverse transcription polymerase chain reaction.” This is a particular method that scientists and medical doctors use to detect the RNA (ribonucleic acid) sequences that comprise the SARS-CoV-2 virus itself.
For this test, a health professional takes a sample of mucus from deep within your nose (i.e., nasopharyngeal swab) or throat. A laboratory then tests the sample for genetic material from the coronavirus itself. That material can only be present if there is an active COVID-19 infection – thus, making the test results conclusive.
How fast is the RT-PCR test?
The swab collection itself takes around 5-10 minutes per person (depending on their level of discomfort). Only a properly equipped laboratory (such Safework Laboratories) can process the samples. Results can be available within 24-36 hours.
Note: Nasopharyngeal swabbing is the more common way of getting mucus samples. But Safework Laboratories is working towards NATA accreditation for saliva and oropharyngeal (throat) PCR testing, which is a faster and more comfortable method for patients compared to nasopharyngeal swab collection.
How accurate is it?
RT-PCR tests are the most reliable of diagnostic tests available, with at least 94% peak accuracy. Unlike antigen tests, PCR tests can detect the coronavirus even at low viral loads and at nearly any stage of the infection.
However, there’s a small chance that false-negative results can occur, if health professionals happen to collect the nasopharyngeal swab only a day or two after a patient acquires the infection. This is because the coronavirus typically takes a few days to replicate itself within the nose and throat. Swabs won’t be able to pick up any of the virus’s genetic material before that happens.
Nevertheless, when used to complement a clinical examination, the RT-PCR test provides the best means for doctors to make a conclusive diagnosis.
PCR vs Antigen Test – the Verdict
After considering all advantages and disadvantages of the PCR vs antigen test, the answer is clear.
PCR testing is significantly more reliable
The PCR test is the more reliable test, as it offers accuracy that will always be superior to antigen testing.
PCR testing has a few additional advantages:
Easier logistics and less time required by health professionals
The rapid antigen test may have a faster total turnaround time (15-30 minutes) and the advantage of being able to process test results on-site. But in practice, that comes at a logistical cost. Each health professional must spend 15-30 minutes per client. This has a direct impact on how many people can be tested.
By contrast, the interaction stage between clients and health professionals for a PCR test is much shorter, running no more than 5 to 10 minutes. That means one health professional can attend to many more people in a day. (After that, the laboratory will handle all collected sample batches and email results back to clients or doctors.)
Thus, widespread PCR testing for large groups of people can be easier and safer for organisations to manage.
Satisfying requirements for travel
Most countries around the world prefer using the RT-PCR test instead of the antigen test as a requirement for entry or travel within their borders. Many require both their citizens and visiting foreigners to show proof of a recent negative PCR test result.
PCR vs Antigen test – Infographic
This summary infographic compares the accuracy of PCR and antigen tests, according to the latest studies.
Safework Laboratories PCR-based COVID-19 testing
This is why Safework Laboratories offer RT-PCR testing as our standard method of analysis for COVID-19 infections.
Our COVID-19 tests are accredited by the National Association of Testing Authorities, Australia (NATA). We currently offer comprehensive PCR testing services for travellers and for employers that want to monitor employee health and safety.
So if you need our help, contact us and schedule your COVID-19 tests with SafeWork Laboratories today.
References:
- Wise, J. (2020). Covid-19: Lateral flow tests miss over half of cases, Liverpool pilot data show. BMJ, m4848. https://doi.org/10.1136/bmj.m4848
- Prince-Guerra, J. et al (2021, January 28). Evaluation of Abbott BinaxNOW Rapid Antigen Test for SARS-CoV-2 Infection at Two Community-Based Testing Sites — Pima County, Arizona, November 3–17, 2020 | MMWR. Centers for Disease Control and Prevention; CDC. https://www.cdc.gov/mmwr/volumes/70/wr/mm7003e3.htm
- Public Health Laboratory Network – Communicable Diseases Network Australia Joint Statement on SARS-CoV-2 Rapid Antigen Tests. (2020, October 9). Department of Health; Australian Government. https://www.health.gov.au/sites/default/files/documents/2021/08/phln-and-cdna-joint-statement-on-sars-cov-2-rapid-antigen-tests_0.pdf
- Interim Guidance for Antigen Testing for SARS-CoV-2 | CDC. (2020, February 11). Centers for Disease Control and Prevention (CDC); United States Federal Government. https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html
- Overview of Testing for SARS-CoV-2 (COVID-19) | CDC. (2020, February 11). Centers for Disease Control and Prevention (CDC); United States Federal Government. https://www.cdc.gov/coronavirus/2019-ncov/hcp/testing-overview.html
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