Since coronavirus began its rampant spread, we've slowly come to learn about the confusing array of available testing options at doctors' offices and health care facilities—many based on mucus and others that use saliva or blood. Given the plethora of options, how do you know which test is right for you? And do they all work the same way? Or tell you the same information?
We will start with the basics. There are currently two primary types of COVID-19 tests: diagnostic tests that look for active coronavirus infection in your mucus or saliva, and blood tests that hunt for antibodies—evidence that your immune system has encountered the infection before.
Let’s say your doctor orders a diagnostic test to confirm or rule out a coronavirus infection. Even among those tests, important differences exist. According to Daniel D. Rhoads, MD, section head of microbiology at Cleveland Clinic, there are a couple of ways to detect SARS-CoV-2, the virus that causes COVID-19. Some tests look for a piece of the coating of the virus—they’re called antigen tests—and other tests detect nucleic acid (such as RNA) belonging to the coronavirus.
RNA tests are highly sensitive. “These tests can remain positive even after somebody is no longer sick and no longer shedding virus that can infect other people,” Dr. Rhoads tells Health.
Antigen tests, by contrast, are generally quick and cheap but often less accurate than RNA tests for detecting the novel coronavirus. The problem is that antigen testing is more prone to false negative results, meaning these tests are more likely to miss cases of active infection. And neither antigen nor RNA testing predicts when someone is no longer contagious, says Dr. Rhoads.
Getting the right COVID test
The right test, then, depends on the goal, such as confirming an active COVID infection; identifying asymptomatic or pre-symptomatic individuals who might be shedding virus, or determining whether someone previously had COVID. “There is not a lone testing approach that is going to meet every need and solve every problem,” Dr. Rhoads points out.
Here’s what you should know about the different types of COVID tests, how they’re used, and what they can tell you.
Molecular test (aka RNA or PCR test)
These diagnostic tests are considered the most sensitive for detecting an active infection, and the results are highly accurate. You might take one if you or your doctor think you have COVID. You might also be asked to take this type of test if you need to prove to your employer or your college that you are not currently infected prior to returning to work or campus
In most cases, a health care provider will collect mucus from your nose or throat using a specialized swab. (Some molecular tests now use saliva, which people may find more comfortable.) Molecular tests are often called PCR tests, short for polymerase chain reaction, the lab technique used to detect the virus’s genetic material, explains the US Food and Drug Administration (FDA). Turnaround time varies from minutes to days or longer, depending on whether the sample is analyzed onsite or sent to an outside lab, explains the Mayo Clinic.
Tests using a nasopharyngeal swab—the one that goes deep into your nose to the back of your throat—are still considered the gold standard. But in recent months, at-home test kits have become available that allow people to collect their own their sample (mucus or spit) and overnight it to a lab for analysis.
These test kits are easy to use and perhaps less intimidating than long-swab testing used in health care settings. Plus, one small study suggests that when people are taught proper technique for collecting their own sample, self-testing yields results that can be just as accurate as those performed by health care workers.
Antigen test (aka rapid test)
This type of diagnostic test is often called a “rapid test” because the turnaround time is much quicker than an RNA test. It’s also cheaper to produce. As a result, antigen tests are being used to screen large numbers of people, like at airports, a recent article in the journal Nature points out.
From a patient’s point of view, antigen testing works in much the same way as molecular testing. Your health care provider will swab the back of your nose or throat to collect a sample for testing. But instead of waiting days for your results, an antigen test can produce a result in an hour or less, says the FDA. If you test positive, it’s probably correct: Antigen tests are highly accurate. The problem is, these tests are more likely to miss active infection. If you have COVID symptoms but test negative, your doctor may order a molecular test just to rule out a false negative.
Some antigen tests can be performed right at your health care provider’s office, meaning you don’t have to go a lab for testing. Sindhu Aderson, MD, of Chicago-based Northwestern Immediate Care, says these “point-of-care” tests are mostly used in emergency departments, doctor's offices, and outpatient clinics.(Note: Not all rapid, point-of-care tests are antigen tests. The FDA in September granted emergency use of Roche’s rapid PCR-based combination test for SARS-CoV-2 and the flu.)
Antibody test (aka serology test or blood test)
This test looks for antibodies to the coronavirus. Antibodies are proteins your immune system produces to fight off a foreign invader, such as a virus. A COVID-19 antibody test cannot diagnose active coronavirus infection. All it tells you is whether you’ve been infected at some point in the past, even if that occurred months ago. Antibodies do not become detectable until at least several days after an infection has started.
There are no FDA-authorized, at-home antibody tests. You’ll have to see a health care professional, who will take a blood sample via a finger prick or a blood draw from a vein in your arm. The vast majority of these tests are performed at a central lab, which can take a couple of days to process. But the FDA just approved the first antibody point-of-care coronavirus test, making it possible for doctor’s offices, hospitals, urgent care centers, and emergency rooms to get an answer within 15 minutes using blood from a person’s fingertip.
Antibody testing isn’t recommended until at least 14 days after the start of symptoms, says the Mayo Clinic. If you test too early—while your immune system is still mounting its defense—it may not provide an accurate result. Sometimes antibody testing is done along with viral testing when someone seeks care late in the course of their illness. It may also help confirm a diagnosis of Multisystem Inflammatory Syndrome in Children, a condition linked to COVID.
If you have antibodies to SAR-CoV-2, does that mean you have immunity against future COVID infections? Researchers still don’t know for sure, but recent studies suggest that past bouts may provide some protection.
Antibody testing has a role to play in the current pandemic because it can provide information on the prevalence of the infection in the population and the frequency of asymptomatic infection, per the FDA.
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