What’s the difference between a PCR COVID test vs. rapid test?
Technically, they’re entirely different tests that look for different aspects of a virus molecule to confirm infection. Because of that, the difference is sensitivity, accuracy, timeand price.
A PCR test detects the RNA found at the center of the SARS-CoV-2 virus particle, Dr. Aronoff explains. This RNA lingers after you’re no longer shedding intact virus particles, so people who have had COVID-19 can be positive by PCR tests for weeks, even months, after they’ve recovered from infection. “A positive PCR test says that a patient has been infected with SARS-CoV-2 but it does not tell us if they are contagious,” he adds.
A rapid antigen test, meanwhile, detects parts of the virus shell that surrounds the RNA, a protein only made when someone is generating an intact, infectious virus, he explains. So, a positive rapid test generally means someone is contagious.
Because PCR tests are more sensitive and can detect traces amount of the virus, they’re generally believed to be more accurate. However, a positive rapid test is highly accurate that you are currently contagious, while a positive PCR test doesn’t necessarily mean you’re contagious.
The other big factor in a PCR COVID test vs. rapid test is time, as a PCR test requires about 2 days to deliver your results, but a rapid test takes just 10-20 minutes.
Lastly, price; home tests should be $10-20, while PCR tests out of pocket usually run $100-120. That being said, the LabCorp Pixel PCR test is currently free for everyone — they either bill your insurance or the federal COVID relief bill — and many community testing sites offer PCR tests for free. So it actually won’t cost you anything to take the more accurate test as long as you choose the right brand.
Is the PCR test for COVID-19 accurate?
Yes, a PCR COVID test is accurate at detecting if you have or had the SARS-CoV-2 virus. But it’s less accurate on whether you’re contagious or not.
PCR tests look for the RNA inside of a SARS-CoV-2 virus particle, and RNA sticks around even after you’re no longer shedding intact virus particles. So a PCR test can stay positive for weeks after you’ve stopped being contagious.
As Dr. Aronoff puts it: A negative PCR test is very reassuring that someone is not infected. A positive PCR tells us that someone is infected but does not tell us how contagious they are. A positive rapid test is very concerning that someone is both infected AND contagious.
How accurate are rapid COVID tests?
It depends on your stage and the intensity of the infection. If you have COVID symptoms and are in the thick of it, a positive rapid test nearly always means you are currently infected and contagious.
However, rapid tests aren’t as sensitive to lower levels of the virus, so if you’re asymptomatic or you’re in the early or late stages of being infected, a rapid test might come back negative even though you are infected and potentially contagious.
Because of this, studies show rapid tests only correctly identify a positive COVID infection 58-72% of the time.
As Dr. Aronoff puts it: A positive rapid test is very concerning that someone is both infected AND contagious. But a negative rapid test is generally not very helpful. You need to either repeat testing every 3 to 5 days for 10 days or take a PCR test to confirm, as PCR tests are more sensitive to low levels of the virus.
Does the accuracy of a COVID rapid test change from brand to brand?
When it comes to rapid COVID tests, no. We definitely don’t know the full story yet, but these tests are all emergency authorized by the FDA, so they should be the same. But they haven’t been as rigorously tested and evaluated as they would in a non-emergency circumstance, and we don’t have studies yet that does a brand-to-brand comparison.
However, for PCR COVID tests, accuracy does change brand-to-brand. We know that each PCR test is measured for sensitivity, with a lower NDU/ml indicating a test is more sensitive and therefore will provide more accurate results. Ideally, you want a test with a sensitivity below 1,000 NDU/ml, but the least accurate tests can run up to 180,000 NDU/mls.
Do rapid tests pick up COVID strains like Omicron?
Yes, but it won’t tell you which strain you have.
While your tests will be able to detect COVID regardless of which strain you have, “there is no current test available to the public that…is able to distinguish between strains,” says Dr. Schwartz.
When you see the CDC and other health organizations reporting how many people currently have the Omicron or Delta variants, these numbers are based on genomic sequencing of positive SARs-COV-2 tests taken from a small sample of hospitalized patients, not the everyday public taking rapid tests or PCR tests, she explains.
Is there an ideal time of day to take a rapid test?
No, you can do both a PCR and a rapid test any time of the day.
More important is that you want to test “when you are symptomatic or within a week of having had a known exposure to someone with a COVID-19 infection,” Dr. Schwartz says.
How long after COVID exposure should I get tested?
If you are exposed to someone who has COVID, wait at least 48 hours (two days) to get tested, but do get tested within seven days, Dr. Schwartz explains. That goes for both a PCR test and a rapid antigen test.
New research published in the New England Journal of Medicine reports that you want to wait at least 48 hours “because it [hasn’t been] enough time for the virus to replicate and be picked up by a molecular test or antigen test.” On the other end of the spectrum, research shows rapid tests are the most sensitive in the first week.
Alternatively, you should take a COVID test once you start having symptoms.
How likely is a false negative rapid COVID test?
A false negative means that the test shows you don’t have COVID-19 when, in fact, you do. We don’t know exactly how common false negatives are, but a 2021 study analysis by Cochrane suggests it happens with about 1 in 4 to 1 in 8 tests for people with symptoms.
A false negative is much more likely if you’re asymptomatic. A review of more than 24,000 test samples before Omicron found rapid antigen tests were only 58% accurate at detecting COVID for people who didn’t have symptoms. That same Cochrane study analysis reported false negatives happen on about 1 in 3 tests in people with no symptoms.
If you’re thinking that number is high, you are correct. That’s why many docs, Dr. Schwartz included, advise that if you take a rapid test and it’s negative, consider taking a PCR test just to confirm. This is especially true if you are symptomatic or have been exposed to a positive person.
“The rapid test may require high viral loads to yield a positive result,” she said, whereas a PCR test is much more sensitive and can detect COVID at lower levels.
What type of at-home COVID test should I take?
Ideally, take a rapid antigen test, and then depending on the results, either more rapid tests or maybe also a PCR test.
Rapid antigen tests are cheap and quick. Although they’re less sensitive, when they do come back positive, it accurately reflects that you’re shedding a lot of inactive COVID viruses. This makes an antigen test a better marker of whether you’re infected and contagious.
The issue is, that when a rapid test comes back negative, you might not actually be negative. Considering the whole point of taking a test is to know if you need to quarantine to minimize spreading the virus, we have to err on the side of caution and say: If you’re only going to take one test, it’s smarter to opt for a PCR test.
Keep in mind that PCR tests can remain positive for weeks after someone is no longer contagious, Dr. Aronoff pointed out during the medical review of this piece. Because of that, you shouldn’t re-take a PCR test after you’ve confirmed you’re infected.
Instead, once you know you’re positive, you can retest with a rapid test after 5-10 days to verify when you’re no longer shedding the virus.