Hour-long queues for coronavirus PCR tests are prompting government officials to push the masses to use rapid home antigen tests instead, but public health experts warn that the 15-minute tests are sometimes prone to false negatives.
Massachusetts Gov. Charlie Baker said last week: “Rapid tests are in some ways a more accurate measure of whether or not someone is actually transmitting COVID than a PCR test.”
Dr. Davidson Hamer, professor of global health and medicine at Boston University, called the governor’s statement “strong.”
Dr. Todd Ellerin, director of infectious diseases at South Shore Health, said that while there is “a grain of truth” in the governor’s statement, “it’s complicated.”
We asked experts to evaluate the accuracy of rapid home antigen tests, PCR tests, and determine when one should be used instead of the other. The answers have been slightly edited for clarity.
How accurate are rapid antigen tests compared to PCR tests?
Your hands: Laboratory-tested PCR results are much more accurate. But if you look at the PCR arc, it can take much longer and often reveals a past infection that can no longer be contagious. When a rapid test is positive, this is probably the point at which a person is most contagious. On the other hand, just because you have symptoms and a negative rapid antigen test does not mean you don’t have COVID. You need to be careful and you need to retake this test several times over several days. Conducting rapid tests several times over a short period of days increases the likelihood of obtaining an accurate result.
Hamer: With rapid tests, if you are positive, there is a good chance that the person may be transmitting the virus at that point in time, but PCR is much more sensitive. It may be detected earlier, but may be positive before the person has enough viral load to transmit, and after the person’s symptoms have resolved and past the point of transmission. However, a rapid test can come back negative before symptoms appear, when the person is still capable of transmitting the virus, so PCR remains the best test at the moment despite its limitations.
Are all rapid tests the same?
Your hands: It is not clear if all rapid antigen tests are created equal, but they all have FDA emergency use clearance, so there is a minimum accuracy they need to prove. But remember, these tests were proven before the omicron. Ideally, we would like these tests to be validated. Until this happens, there is a high chance that these tests are free of infections. It is also important that people read the instructions carefully.
Hamer: Quite a few have been tested and approved by the FDA for emergency use. They are not all the same, but they are similar in their ability to detect an infection in someone with symptoms. A study of about 80 home antigen tests found that the accuracy was about 100% in cases where patients had a high viral load but it dropped off fairly quickly. This means that asymptomatic patients or those with a low viral load may still be transmitting the virus, but a rapid test may not detect their infection.
When should I get a rapid coronavirus test compared to a PCR test?
Your hands: The good news about the rapid test is that you get results quickly, so if you don’t have access to PCR, you can still get an idea. However, there are concerns about missed infections, especially when it comes to omicrons and any other options that come after it. The best time to use PCR is when you can get results quickly.
Hamer: If someone is showing symptoms and wants a quick response, an antigen is the way to go, especially if you don’t have access to a PCR test. Antigen tests are more likely to be accurate if you have symptoms and have been exposed within the past 2 to 5 days. PCR tests are good for population-based surveillance, such as group testing in schools, where early detection of infections is the key to success. PCR is really useful if there is a short processing time of less than 24 hours and low cost.
Do I need to take a rapid antigen test before attending events?
Your hands: In an ideal world, yes. But for this to work, people will test a couple a couple of days before and again right before. Unfortunately, there are currently not enough available rapid antigen tests for this.
Hamer: I’m not sure if this can be used as an effective validation tool for participation in events. If someone is contagious but has no symptoms, the likelihood that an antigen test will find someone contagious is low.