We hear about them in the news every day: rapid antigen tests, and molecular/PCR tests. Everyone seems to have a different opinion about which test is best, when to get tested and why we need different types of tests. In this Q&A, three Roche Diagnostics experts share insights and clarity on the topic.
For diagnosis – to know if you have the virus – there are two types of tests, antigen tests and nucleic acid amplification tests (NAAT). Polymerase chain reaction (PCR) tests are a type of NAAT test. These types of tests have been around for years.
PCR tests, which are also called molecular tests or viral RNA tests, detect genetic material from the virus. Most PCR tests require a lab and special equipment, and typically give results in 24 hours. Since 2014, PCR tests known as rapid PCR have been commercially available, and can give results in 20 minutes. That’s why they’re known as the gold standard.
Antigen tests, also called rapid tests or rapid antigen tests, identify portions of the virus’s proteins (antigens). Antigen tests often use lateral flow, a technology that’s similar to what’s used in pregnancy tests and rapid strep tests, and can give results in about 15 minutes.
Bottom Line: PCR and antigen tests have been used in labs for years to detect many pathogens. Both tests can yield rapid results or be performed in the lab.
If you don’t have any symptoms, PCR is the best test to use. However, if you’re within a week of symptom onset, antigen tests can give you a quick result before you go to work or an event, or if you’ve been exposed to the virus or have COVID symptoms, said Jamie P. Deeter, a molecular virologist who specializes in coronaviruses and a scientific advisor at Roche Diagnostics, where she has helped launch three SARS-CoV-2 diagnostic tests.
PCR tests provide extremely accurate results. Use them when it’s important to minimize the risk of infecting others, or if you want to be extremely confident about your diagnosis – for example, before visiting vulnerable loved ones or going to a hospital. You can also take a PCR test to follow up on your negative antigen test result.
View a chart that compares antigen and PCR tests.
Bottom line: You get to choose which test is right for you. Consider your comfort level with test accuracy and how quickly you need results.
Both PCR and antigen tests usually start with a nasal swab. When you take an at-home antigen test, you collect the sample and administer the test yourself. “It’s important to use at-home tests as they were intended to be used,” said Deeter. Studies have shown that the human factor can impact the accuracy of antigen tests. “The further you take tests from a lab setting, the more issues you have with proper sample collection and use,” added Allyson Kozak, a scientific liaison at Roche Diagnostics with expertise in SARS-CoV-2 antibodies. “So if you decide to take a test at home, read and carefully follow the instructions.”
Bottom line: If you’re taking an at-home antigen test, be sure you follow the directions and use it properly. If you’re worried about accuracy, go with a lab-based test.
In the United States, only certain tests have received Emergency Use Authorization from the Food and Drug Administration. In some areas there haven’t been enough tests available to meet demand. A push to increase testing availability and expand the government’s free testing program has begun to improve the situation.
Bottom line: The U.S. government has prioritized access to testing. That’s good news for everyone.
PCR tests (whether rapid or performed in a lab) are so sensitive they can detect very small amounts of virus in a sample, and they focus specifically on the genetic material of SARS-CoV-2, the virus that causes COVID-19. That gives them about 98% sensitivity. They detect almost every active infection, and it’s rare for someone uninfected to receive a positive test.
With rapid antigen tests, the tradeoff for speed is sensitivity. Most antigen tests have a sensitivity of anywhere between 50% and 90%.
Studies show antigen tests are most effective when people are tested in the week after showing symptoms. The journal Nature reports that if a person has low amounts of virus in their body, an antigen test might give a false-negative result.
Bottom line: Data show PCR tests are the most accurate option.
“Antigen tests can catch the sickest of the sick,” Kozak said. “Those people have the highest viral loads and are the greatest threat to the public.” But typically, after the first few days of symptom onset, most people’s viral load drops substantially, and the tests are less sensitive. They may not result in a positive even though a person is positive. That’s why Kozak has concerns about using antigen tests to screen people who don’t have symptoms. “Antigen tests are convenient, and they’re cheaper, but they only catch about half the people who are positive. They may miss those lower viral loads, and people who don’t have symptoms could still transmit the virus.”
Both PCR and antigen tests can tell you if pieces of the virus are present, but neither indicates an active infection, Deeter said. Antigen-based tests are sometimes considered good indicators of people who may be transmitting the virus because they can identify people who are at the peak of infection, when virus levels in the body are likely to be high. That said, data don’t support the use of any of these tests to indicate if a person is able to transmit the virus. “Many people see antigen testing as an essential tool in the fight against COVID,” Deeter said, “but antigen tests must be used appropriately and in conjunction with more sensitive tests.”
Bottom line: When used properly on people at the peak of infection, antigen tests may be better at indicating infectiousness.
Experts believe that a robust national plan for vaccination and COVID-19 surveillance, including greater access to testing, will help get America out of the pandemic. Testing helps us see where the virus is and who’s infected with it so we can take measures to control it.
“Containing the virus is all about identifying it and quarantining people who have the disease,” Deeter said. “That’s why testing is so important. Different types of tests have risks and benefits. It’s just a matter of understanding which tools are best suited for which use.”
Deeter believes using PCR and antigen testing in tandem could be a sound strategy. “We can marry the two for symptomatic patients,” she said. “We just need to be aware of the risk of false negatives with antigen tests – and the need to follow up on them with more testing.”
What’s most important is what we do after testing, Deeter said. “If we test positive, or someone in our household tests positive, we need to follow the guidance so we don’t infect other people. It’s not OK for your child to play in the football game if a sibling tests positive for COVID.”
Bottom line: Testing will help get America out of the pandemic.
Have more questions about COVID-19 testing? See our continued Q&A.
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Allyson Kozak, Ph.D., MBA, NRCC, is a board-certified clinical chemist and specializes in clinical chemistry, toxicology and immunoassays, including SARS-CoV-2 antibody diagnostics at Roche Diagnostics. She has also served as a laboratory medical director, toxicology expert witness and assistant professor at Case Western Reserve University School of Medicine.
Jamie P. Deeter, MS, Ph.D., obtained her doctorate in infectious diseases, specifically studying viral evolution related to coronaviruses. Previously she has been an ORISE fellow at the CDC, served as a director of research for a small biotech firm and joined Roche’s medical and scientific affairs team four years ago. During the pandemic she has helped launch three of Roche’s SARS-CoV-2 diagnostic tests and has served as a subject matter expert.
