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Should I take a rapid coronavirus test or a PCR? Public health experts breakdown the facts

Boston Herald - 1/17/2022

Jan. 18—Hours-long lines for coronavirus PCR tests are prompting public officials to push the masses to use rapid, at-home antigen tests instead, but public health experts warn the 15-minute tests are sometimes prone to false negatives.

Gov. Charlie Baker last week said: "Rapid tests, in some ways, are a more accurate measure of whether or not somebody 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 disease at South Shore Health said while there's "some truth" to the governor's statement, "it's complicated."

We asked the experts to break down the accuracy of rapid at-home antigen tests, PCR tests and lay out when one should be used over the other. Answers have been edited slightly for clarity.

How accurate are rapid antigen tests compared to PCR tests?

Ellerin: PCR results, which are tested in a lab, are much more accurate. But if you look at the arc for a PCR, it can last much longer and is often picking up a past infection that may no longer be infectious. When a rapid test is positive, that's probably the point when a person is most infectious. On the other hand, if you are symptomatic and test negative on a rapid antigen test, that does not mean you do not have COVID. You need to stay cautious and you need to retake that test several times over a couple of days. Taking rapid tests multiple times over a short period of days increases the likelihood of an accurate result.

Hamer: With the rapid tests, if you are positive, then there's a good likelihood that a person is able to transmit the virus at that point in time, but the PCR is much more sensitive. It can detect earlier, but may be positive before a person has enough viral load to transmit and also after a person's symptoms have resolved and past the point of transmission. The rapid, however, may come back negative before the onset of symptoms when a person is still able to transmit so the PCR remains the best possible test right now, despite its limitations.

Are all rapid tests created equal?

Ellerin: It's unclear if all rapid antigen tests are created equal but they all have an Emergency Use Authorization from the FDA, so there's a minimum accuracy that they needed to prove. But remember, these tests were proven pre-omicron. Ideally, we'd like these tests to be validated. Until that happens there's a high probability these tests are missing infections. It's also important people carefully read the instructions.

Hamer: A pretty large number are FDA reviewed and approved, under the Emergency Use Authorization. They're not all not the same, but they are similar in their ability to detect an infection in somebody who is symptomatic. A study of roughly 80 at-home antigen tests found the accuracy is near 100% in cases where patients had a high viral load, but it drops off pretty quickly. That means patients who are asymptomatic or who have a low viral load may still be transmitting, but their infection could be missed by a rapid test.

When should I take a rapid coronavirus test versus a PCR test?

Ellerin: The good news about the rapid test is you get results back quickly, so if you don't have access to a PCR, you can still get an idea. There is the concern over missed infections, however, particularly when it comes to omicron and whatever variant comes after that. The best time to use a PCR is when you can get the results back quickly.

Hamer: If somebody develops symptoms and they want an answer quickly, antigen is the way to go, especially if there's no access to a PCR test. Antigen tests are more likely to be accurate if you're symptomatic and had an exposure in the last 2 to 5 days. PCR tests are good for population-level surveillance like the pooled testing happening 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.

Should I take a rapid antigen test before attending events?

Ellerin: In an ideal world, yes. But for it to work, people would be testing couple a couple of days before and again right before. Unfortunately, there isn't enough accessibility to the rapid antigen tests right now for that to happen.

Hamer: I'm not sure if this can be used as an effective screening tool to go into events. If someone is infectious but has no symptoms, the likelihood of an antigen test finding someone infectious is low.

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