WASHINGTON — As COVID-19 cases skyrocket to the highest numbers since the start of the pandemic, it’s become increasingly difficult for many Americans to track down a COVID-19 test.
Then when you finally get a shot at one, you’re faced with deciding which kind of test you need – a molecular PCR or a rapid antigen? The answer depends on the question you're asking the test.
In what situations should you get a PCR test? What about a rapid antigen test?
If you need to know with absolute certainty whether you have COVID-19, a PCR test is the most reliable. If you are asymptomatic and want to check whether you are contagious, take a rapid antigen test.
WHAT WE FOUND
The major difference between the two tests has to do with the specificity and sensitivity of the tests. Specific meaning if you get a positive result, what’s the chance you actually have the virus? And sensitive meaning, if you have the virus, what’s the chance you get a positive result?
Antigen tests, while highly specific, are slightly less sensitive than a molecular PCR test. PCR tests are so sensitive in fact, that they can sometimes pick up on the smallest amount of virus before or after your period of contagiousness.
“We know that the PCR tests, those can stay positive for weeks after someone recovers because they're so sensitive,” Dr. Morice says. “They'll pick up remnants of the infection, whereas antigen tests probably don't do that.”
Our experts say the test you take depends entirely on the question you are asking it. Do you want to confirm that what you have is COVID-19, or are you checking to see if you can safely go out to a concert or to dinner?
“The antigen test is a great tool because you're asking the test to tell you whether or not you're contagious, whether you're a danger to others,” Dr. Adalja explains. He says a PCR should be used by someone who needs to know whether what they have really is COVID-19.
“They're asking ‘what am I sick with?’ Or ‘why am I sick?’” Dr. Adalja says, “versus just screening to see ‘am I safe to be around people?’”
Dr. Gronvall emphasizes that timing is going to be very important here. If you take an antigen test to know whether you can safely go to dinner, a test taken that morning won’t give you a reliable answer.
“Tests are a moment in time,” Dr. Gronvall says. “If you are using these rapid antigen tests to reduce the risk of getting together with other people, take the test as close to the event as you can.”
Both tests are highly accurate and will tell you whether you have the virus or not. So in this period when a test can be difficult to track down, using any you can find to slow down the spread will be valuable.