Here’s When You Should Take a PCR or a Rapid Antigen Test

People wait in line for COVID-19 tests.
Eva Marie Uzcategui/Bloomberg via Getty Images
  • With Omicron surging in the United States, many people are taking COVID-19 tests to see whether they test positive.
  • Most people will take either a rapid antigen or PCR test.
  • Though the PCR test is considered to be the most accurate, there is a time and a place for an antigen test as well. It is a smart tool to have in the medicine cabinet.

As Omicron continues to spread in the United States, many people with and without symptoms use rapid and PCR tests to determine whether they have COVID-19.

But the Omicron variant and its quick spread have complicated matters as testing supplies are limited, and some people report that they have tested negative despite having COVID-19.

So, when is negative actually negative, and when is it potentially a false negative? When should we see a doctor to administer a test, and when will an at-home test suffice? 

Types of tests

There are two types of COVID-19 tests — diagnostic and antibody. The diagnostic tests are designed to show whether you have active COVID-19 disease, while antibody tests show whether you’ve had COVID-19 in the past.

Also, within the diagnostic tests are categories. These include molecular and antigen tests, more commonly known as PCR and rapid tests.

These are designed to detect an active SARS-CoV-2 infection, but they do not work precisely the same way. Timing and level of disease both play an essential role in how and when the tests should be used.

“The general public has difficulty in understanding that the interpretation of COVID tests is not a simple positive or negative. It is more complex and depends both on the type of test and the timing of when the test is done relative to the time of exposure,” said Dr. William Schaffner, professor of Preventive Medicine, Department of Health Policy, and professor of Medicine, Division of Infectious Diseases at Vanderbilt University Medical Center.

To understand what test does what and when you might need to take which, let’s take a look at what each does.

What is the difference between a PCR and a rapid test?

Rapid antigen tends to be the test for over-the-counter or at-home COVID-19 tests.

You can buy them in most pharmacies. These tests come in handy when there are more virus particles in the system — when people are likely to pass the virus to another. 

PCR tests, which are still typically administered by medical professionals, are much more accurate because they are more sensitive than antigen tests. PCR tests can determine whether the body has a much smaller amount of the virus and can also determine whether you’ve contracted the virus much sooner than an antigen test.

Which is better: PCR or Antigen?

The answer is complicated. Both tests are helpful in particular situations, but they also have their drawbacks. If we are looking for a bottom line “most accurate,” then that would be the PCR test.

“The PCR COVID test is the gold standard. It is excellent in diagnosing COVID when you are developing symptoms,” said Schaffner. 

“However,” he added, “because it can detect viral fragments, or pieces of the virus rather than the whole virus, it can remain positive for weeks, long after the person has recovered and no longer is contagious. Thus, it is not ideal for determining the end of isolation or quarantine.”

Simply put, you can fully recover from COVID-19 and still test positive for the virus, far from helpful when you want to return to work, travel, or generally assume daily life.

Similarly, the antigen test has specific drawbacks as well. They are decidedly less accurate than PCR tests when a person has a lower amount of the virus in their system. The virus is still present but may still be too low to detect. A person can have small amounts of the virus and test negative with an antigen test, which is why they are not the most reliable in determining whether a person has COVID-19.

“The rapid test has the obvious advantage of having results available within 15 minutes, but it is not as sensitive as the PCR. That is, it can register as negative when the person still is shedding small amounts of virus,” said Schaffner. “This is particularly true within the first couple of days after exposure when there still is not enough virus in the nose to turn the test positive. Nonetheless, the rapid test can be useful when gathering with vaccinated family and friends to provide an additional measure of comfort and reassurance.”

Simply put, a positive test, whether PCR or antigen, should be trusted as a positive test. A negative PCR test should be trusted as negative. A negative antigen test may need to be confirmed with a PCR test.

When should you use either test?

While the PCR test is considered to be the most accurate, there is a time and a place for an antigen test, too. It is a smart tool to have in the medicine cabinet.

An antigen test is helpful for those who are asymptomatic or have minor symptoms but may have been exposed to COVID-19. 

If a patient is asymptomatic and is looking to test whether they have been exposed, they can take two antigen tests 5 days apart. If both are negative and they remain asymptomatic, that is probably an indication the patient really does not have the COVID-19, said Dr. Ting Ting Wong, an infectious disease specialist at NewYork-Presbyterian Medical Group Brooklyn.

But if a patient begins to exhibit symptoms, and the antigen test comes back negative, the best course of action is to confirm with a PCR test. A negative antigen test does not necessarily mean that you won’t transmit the virus.

“Antigen tests miss about one-third of true infections,” said Wong. “A negative antigen test [with symptoms] may mean the test was not as sensitive enough to detect actual virus in the body.”

What is a NAAT test?

What if an antigen test isn’t sensitive enough and a PCR test is too sensitive? Is there a happy medium? The short answer is yes. This is the NAAT test. NAATs, according to the Centers for Disease Control and Prevention (CDC), are high-sensitivity, high-specificity tests designed to detect the nucleic acid and genetic material of the virus. Unlike PCR tests, NAAT results can be obtained as quickly as rapid tests. Most NAAT tests, like PCR tests, have to be done by medical professionals, but some point-of-care tests can have results within 15 minutes.

Wong would recommend her patients obtain the rapid NAAT tests instead of antigen tests — they are more accurate and come with the convenience of returning results quickly. That said, people will still have to seek out NAAT tests at clinics or doctor’s facilities because they are not currently available in-home.

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