Reviewed by: Carrie A. Horn, MD, chief medical officer and chief of the Division of Hospital & Internal Medicine at National Jewish Health; and Rheeti Khare, PhD, director of the Infectious Disease Laboratory at National Jewish Health
February 25, 2022
Antigen? PCR? IgM? IgG? All these COVID testing terms might be confusing, but learning the terminology is essential for determining which COVID test is right for you and your family, especially as the virus continues to fluctuate. As our understanding of the virus grows, caregivers are better equipped than ever to combat the disease. Our experts at National Jewish Health offer these tips to help you better understand the variety of testing options available.
The need for testing was much greater early in the pandemic, when vaccines weren’t available. Back then, experts recommended COVID testing if you were exposed to someone who was positive or if you’d been in a high-risk situation, such as a crowded space. Social distancing and constant testing were important for slowing the spread. And, because of this, you may be wondering if it’s okay to venture back into everyday life without getting tested at every turn. Can you go to your kid’s graduation? What about that baseball game? That forever-postponed party with friends and family?
In most cases, the answer, quite simply, is yes. “I don’t know that there’s a lot of value in testing asymptomatic people at this point,” said Carrie Horn, MD, chief medical officer at National Jewish Health. “But there are times when it is still important.”
Dr. Horn explains that if you have symptoms, testing is still crucial, along with social distancing, and even if you’re asymptomatic, there are certain scenarios that warrant getting a test.
“If you have someone in your home who is extremely high-risk, and you want to be more careful around this person, then testing is reasonable,” said Dr. Horn. “In addition, if you are about to travel or are headed to a large indoor gathering, testing beforehand can provide reassurance.”
If you believe that you or a loved one might be in one of these high-risk circumstances, you can find information on getting the right COVID test for you below.
If convenience and cost are your top priorities, you might consider using an antigen test. While not as accurate as PCR tests (explained below), antigen tests are more affordable and can deliver results at home within 15 minutes. For instance, the free tests recently distributed to households by the U.S. government fall under this category.
COVID tests typically focus on one of two targets: the protein “shell” on the outside of the virus or the genome on the inside of the virus. Imagine you’re shopping for a new car. Two important things to identify the make and model of a vehicle would be the body (the protein “shell”) and the engine (the genome). Antigen tests focus on the outside of the virus just like you would begin your car search by looking for an appealing color and size. Antigen tests do this by using antibodies (part of our immune system’s defense against foreign invaders) to target specific proteins.
Reeti Khare, PhD, director of the Infectious Disease Laboratory at National Jewish Health, explains how this test conveys results by describing how antibodies react to viral proteins.
“Antibodies are the best system for binding to a protein,” she says. “We essentially purify antibodies that are specific to that one protein. When you have enough tagged antibodies bind, you see a color change that is visible to the naked eye.”
PCR tests are the most accurate tests currently available for COVID. Unlike antigen tests, PCR tests are able to target the genome, the virus’s engine.
Using changes in temperature, a PCR test can cause parts of the virus to duplicate. This amplifies the presence of the virus and reveals the genome. “It’s like hitting the enhance button on your phone. You see this tiny thing, and you enhance, enhance, enhance,” said Dr. Khare. “That’s why PCR works better than antigen.”
The level of precision makes PCR tests extremely reliable. The main drawback is that the test requires special equipment and expertise, so PCR tests can’t be performed at home. Results tend to take one to three days.
PCR tests use big temperature changes to amplify the virus, and all that heating and cooling takes time. According to Dr. Khare, “Rapid PCR tests use slightly different technologies to get around the heat-up, cool-down process used in regular PCR tests. That’s why you can get results back in 30 minutes.”
However, the speed of a rapid PCR test does come with a cost, as these tests can be more expensive than standard PCR tests and have a slight drop in test accuracy.
Rapid PCR tests are ideal if you have an urgent need to know your COVID status. When seeking one of these tests, your best bet is to find a professional testing facility.
Do you really need to know if you’ve had COVID in the past? Dr. Horn said that beyond giving people extra peace of mind, not really. Generally, post-vaccination, testing for previous COVID infection isn’t that useful.
There are a few specific instances where post-COVID antibody tests might be helpful. For instance, for people who continuously experience acute infection from the virus (despite being vaccinated), an antibody test can determine whether the body is mounting an appropriate defense. The lack of a natural defense might be a sign that a patient is immunocompromised.
When it comes to looking for antibodies, doctors can use either IgM or IgG tests.
IgM is the antibody that your immune system makes immediately when you get sick. This makes the IgM test appropriate for determining very recent infection (around three to 10 days after infection).
IgG antibodies develop later and are your lasting protection. When you get exposed to something a year later and your cells wake up, IgG is what they’re going to release to help you with that repeat exposure.
Unfortunately, no. Dr. Horn said that while a few select PCR tests have been able to indicate Omicron infection, their accuracy is unknown. There’s also no way to test if a person had a specific variant in the past.
According to the Food and Drug Administration, health care providers should treat patients who have COVID-19 based on the patient’s diagnosis and symptoms and not based on the strain of virus. Therefore, tests that identify specific variants are largely unnecessary for most people.
Yes, there is a test for this, but there is only one instance when it would be helpful. If you’ve had a severe reaction to a dose of the vaccine and you don’t want to get a booster, you may want to check your COVID antibody levels. In this situation, your doctor could order an IgG test for specific antibodies that target the nucleocapsid (the part of the virus that helps to package the genome). The reason this test is able to pinpoint COVID infection antibodies, as opposed to those from the vaccine, is that vaccine antibodies target the spike protein, whereas COVID infection antibodies target both the nucleocapsid and spike protein.
If you’re vaccinated, constant testing because of possible exposure is not necessary.
Dr. Horn recommends that you use your best judgment regarding testing and advocates getting vaccinated as one of the best ways we can protect both ourselves and the vulnerable and immunocompromised.
To learn more about National Jewish Health and COVID, check out our prevention guide here. You can also review this chart detailing the differences between COVID tests.
The information on our website is medically reviewed and accurate at the time of publication. Due to the changing nature of the COVID-19 pandemic, information may have since changed. CDC.gov and your state’s health department may offer additional guidance.