Hour-long queues for coronavirus PCR tests prompt government officials to pressure the masses to use rapid antigen tests at home instead, but public health experts warn that 15-minute tests are sometimes prone to false negatives.
Governor Charlie Baker said last week: “In some ways, rapid testing is a more accurate measure of whether someone is actually transmitting COVID than a PCR test is.”
Dr. Davidson Hamer, BU professor of global health and medicine, called the governor’s statement “strong”.
Dr. Todd Ellerin, director of infectious diseases at South Shore Health, said that although there is “some truth” in the governor’s statement, “it is complicated.”
We asked the experts to break down the accuracy of rapid antigen tests at home, PCR tests and determine when one should be used over the other. The answers have been edited a bit for the sake of clarity.
How accurate are fast antigen tests compared to PCR tests?
Your hands: PCR results, which are tested in a laboratory, are much more accurate. However, if you look at the arc of a PCR, it can last much longer and often picks up a previous infection, which may no longer be contagious. When a quick test is positive, it’s probably the point where a person is most contagious. On the other hand, if you are symptomatic and test negative on a rapid antigen test, it does not mean you do not have COVID. You need to be careful and you need to take that test again several times over the course of a few days. Taking quick tests several times over a short period of days increases the likelihood of an accurate result.
Hammer: With the quick tests, if you are positive, then there is a good probability that a person is able to transmit the virus at that time, but PCR is much more sensitive. It can detect earlier but can be positive before a person has enough viral load to transmit and also after a person’s symptoms have disappeared and past the point of infection. However, the rapid may return before the onset of symptoms when a person is still able to transmit, so PCR remains the best possible test right now, despite its limitations.
Are all quick tests created equal?
Your hands: It’s unclear if all rapid antigen tests were created equal, but they all have an emergency use permit from the FDA, so there is a minimum accuracy that they should prove. But keep in mind that these tests were proven before omicron. Ideally, we would like these tests to be validated. Until that happens, there is a high probability that these tests are missing infections. It is also important that people read the instructions carefully.
Hammer: A fairly large number have been FDA reviewed and approved under the emergency use permit. They are not all alike, but they are alike in their ability to detect an infection in someone who is symptomatic. A study of about 80 antigen tests at home showed that the accuracy is close to 100% in cases where patients had a high viral load, but it drops quite quickly. This means that patients who are asymptomatic or who have a low amount of virus may still transmit, but their infection may be overlooked by a quick test.
When should I take a rapid coronavirus test versus a PCR test?
Your hands: The good news about the quick test is that you get results back quickly, so if you do not have access to a PCR, you can still get an idea. However, there is concern about unanswered infections, especially when it comes to omicron and what variant comes after it. The best time to use a PCR is when you can get the results back quickly.
Hammer: If someone develops symptoms and they want a response quickly, antigen is the way to go, especially if a PCR test is not available. Antigen tests are more likely to be accurate if you are symptomatic and have had an exposure within the last 2 to 5 days. PCR tests are good for monitoring at the population level as the overall test that takes place in schools where the key is to try to catch infections early. PCRs are really useful if there is a short turnaround time of less than 24 hours and the cost is low.
Do I need to take a quick antigen test before attending events?
Your hands: In an ideal world, yes. But for that to work, people would test couples a few days before and again just before. Unfortunately, there is not enough availability for the rapid antigen tests right now for that to happen.
Hammer: I’m not sure if this can be used as an effective screening tool to go into events. If someone is contagious but has no symptoms, the likelihood of an antigen test finding an infectious is low.
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