PCR? Antigen? Antibody? Your guide to the different kinds of coronavirus tests and how accurate they are

---

For a virus that was unknown to the world less than a year ago, the sheer number of tests today that detect traces of SARS-CoV-2 — the coronavirus that causes COVID-19 — is mind-boggling.

The federal Food and Drug Administration has so far approved 170 different diagnostic tests and 47 blood tests for the virus. The tests were developed by private companies, government agencies, university labs and individual hospitals. Some are used just in one place — think of a hospital using its own test to run screenings for its patients and staff. And some are deployed nationwide as part of mass testing campaigns.

But the tests don’t all look for the same things. And as newer, faster coronavirus tests come to market, it is becoming increasingly important to understand these differences and learn which test is right for which situation.

So, we spoke with Emily Travanty, the interim director of the Colorado Department of Public Health and Environment’s state public health lab, to get a rundown of all the crucial details.

Know your test

There are two main kinds of coronavirus tests, and they can be roughly separated by the stuff they take from your body in which to look for the virus.

Blood tests, also known as serological tests, scan a blood sample for signs of the virus (this means they involve needles or at least a finger prick to draw your blood). They are looking for antibodies — indicators that your body has produced an immune response to the virus. While these tests can sometimes detect active infections, they are primarily used to see whether you have previously been infected by the virus. They will not tell you whether you are contagious.

Diagnostic tests use all sorts of other bodily gunk that isn’t blood — snot, sputum, saliva — to look for signals of active infection. These are the tests that use Q-tip-like swabs and are commonly available at drive-up and community testing locations around the state. They tell you whether you are infectious. If you think you are sick or have been exposed to the coronavirus, these are the tests you need to get.

Be specific (and sensitive)

When assessing the accuracy of a test, there are two important concepts to keep in mind.

Sensitivity is the test’s ability to accurately detect the presence of the virus if it is there. The less sensitive a test is, the more likely it is to deliver a false-negative.

Specificity is the test’s ability to accurately rule out the presence of the virus if it is not there. The less specific a test is, the more likely it is to deliver a false-positive.

Ideally, tests would be both incredibly sensitive and incredibly specific. But, in practice, there is usually some level of trade-off.

More on antibody tests

There are a bunch of places in Colorado currently offering antibody tests, and the appeal is pretty obvious. Who wouldn’t want to know if that scratchy throat from a few months ago was really COVID-19? They can also deliver more than peace of mind. Epidemiologists use the tests to study how widely the virus has spread.

But the tests have also faced a lot of skepticism. This is because antibody tests tend to be very sensitive but not as specific.

The FDA has published data on the performance of a number of the approved antibody tests. Most all of them have sensitivity values near 100% — meaning if you do, in fact, have SARS-CoV-2 antibodies, the test will find them. The specificity numbers for most of the tests are typically lower, usually in the mid-90s.

That’s not that big of a dropoff. But when magnified by the overall low prevalence of the virus that we currently have, it can start adding up to big numbers of false-positives. According to the FDA numbers, some of the approved antibody tests have positive predictive values of under 60%. That means, if those tests say you have coronavirus antibodies, it’s only slightly better than 50/50 that you actually do.

Let’s get chemical

As the world increasingly tries to walk the tightrope of managing the virus while also reopening economies, there are a lot of people who need a lot of tests to tell them whether they have the virus and are infectious. Think: Teachers, schoolkids, medical professionals, workers, athletes, and anybody else who wants to do normal-ish things with the reassurance that they and others near them are likely virus-free. This is why diagnostic tests are where the real action is right now.

For instance, Illinois-based Abbott just released a $5 “lateral flow” coronavirus test called BinaxNOW, which Travanty likened to the familiar pee-on-a-stick pregnancy tests. (But without the pee; they use nose swabs.) The company claims the test can deliver results in 15 minutes with 97.1% sensitivity and 98.5% specificity.

But Travanty said faster doesn’t necessarily equal better. There are multiple kinds of diagnostic tests. What test you should get — and what you do with the results — depends on your circumstances.

PCR tests

These are the gold standard for coronavirus tests, Travanty said. They hunt for bits of the virus’ genetic material — PCR stands for polymerase chain reaction, which is the mechanism by which that genetic material is amplified to detectable levels.

“They’re capable of picking up very, very small amounts of virus, so they are more sensitive,” Travanty said.

Almost all the diagnostic tests being run in Colorado and reported to state health officials currently are PCR tests, Travanty said. PCR tests are what are offered at all of Colorado’s community-based testing sites.

These tests are most associated with nasopharyngeal swabs — those extra, extra-long Q-tips that go way up your nose. But they can also be run with samples taken from less intense nose, throat or cheek swabs or even from spit.

The challenges with these are, by now, well known. They can take days to process. Supplies and lab capacity to run them have at times been limited. They are only as good the sampling and handling practices of the people who administer them, which has been a concern.

But, because they are the most accurate diagnostic test available, Travanty said these should still be the test of choice for people who are either feeling sick or think they may have been exposed. (Note that if you do think you’ve been exposed to the coronavirus, health officials say you should isolate at home for a week before seeking a test. The virus needs time to build up, and getting tested too soon increases the chances of receiving a false-negative.)

Antigen tests

These tests look for the coronavirus by hunting for its antigens, which are specific proteins on the surface of the virus. They typically use nose swabs.

Travanty said antigen tests are highly specific, so they are unlikely to deliver false-positives. But they are not as sensitive as PCR tests, meaning they can give false-negatives to people who are actually infected.

The tests are most effective in people who are either symptomatic or near the peak of their infection — meaning their best use is to stop people who are most likely to spread the virus. And they are also faster than PCR tests.

For these reasons, Travanty said antigen tests are most valuable for people who are being tested repeatedly — people who work in hospitals or nursing homes, for instance. People who get one-off antigen tests should confirm negative results with a PCR test in order to be certain, she said.

A word about “rapid” tests

That word is: Caution.

The past couple of months have seen a wave of faster, more consumer-friendly coronavirus tests come to market. These rapid tests may be forms of antigen tests or may use other methods.

But Travanty said they should be treated the same way that other antigen tests are. They are useful if you are being tested repeatedly, and they are good at identifying whether the raging cold you have is COVID-19. But for people who aren’t showing symptoms, a PCR test is the best choice.

For example, Travanty mentioned one new rapid test that is one to two statistical orders of magnitude less sensitive than a PCR test. So, caution: False-negatives may abound.

Find a test

Unlike the early days of the pandemic, there are lots of ways to get tested now — from hospitals and doctors’ offices, to drive-up testing sites to home-delivered PCR test kits that you mail back. The best way to go about getting tested is to call your health care provider and ask what they recommend.

But the state also maintains an interactive map of testing locations, often with information about whether the sites offer diagnostic tests, antibody tests or both. Keep in mind that for most if not all of these sites, the diagnostic tests will be PCR tests.Find the map here and zoom or search for the location you want.