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Last updated February 1, 2021. 10 minute read

Your guide to PCR, antigen, and antibody COVID-19 tests

When choosing a COVID test, you need to think about what information you’re looking for. Do you want to see if you have the virus now or whether you had it in the past? PCR and antigen tests let you see if you currently have COVID, while an antibody test tells you if you’ve had COVID in the past. Read on for more.

Dr Chimene Richa Md Written by Dr. Chimene Richa, MD
Reviewed by Dr. Yael Cooperman, MD

When it comes to getting tested for coronavirus disease 2019 (COVID-19), it can feel overwhelming navigating the mountains of information out there. Don’t worry, we’re here to answer all your questions and more.

COVID-19 tests can be divided into two broad categories: diagnostic tests, which tell you if you currently have COVID-19, and antibody tests, which show if you’ve had it in the past. Unlike diagnostic tests, antibody tests give you no indication of whether you are infected at the time you took the test. Let’s dive deeper and break down the important differences between these tests. 

Vitals

  • The two main options for COVID-19 testing are PCR and antigen tests, which can tell you if you have the virus.
  • Antibody testing can tell if you’ve had a COVID-19 infection in the past. It does not tell you if you’re currently infected.
  • The accuracy of the different tests depends on which one you use, whether it’s done properly, and other factors.
  • Results from PCR tests can take anywhere from hours to days, while results from antigen and antibody tests take around 15-30 minutes.

What are the different options for COVID-19 testing? 

The most commonly used diagnostic tests are (La Marca, 2020):

  • Polymerase chain reaction (PCR) tests: These look for the presence of viral genetic material 
  • Antigen tests: These determine whether you have any viral proteins in your system (like the characteristic spike proteins seen on the virus’s outer shell)

Alternatively, antibody tests look for antibodies made by your immune system in response to a SARS-CoV-2—the virus that causes COVID-19 infections. These tests indicate if you’ve had a past infection, but cannot be used to diagnose you with COVID-19.

What are PCR tests and how do they work?

PCR tests, also called molecular tests, look for the virus’s genetic material. All living things—humans, insects, mushrooms, bacteria, and even viruses—have genetic material that is specific to that particular organism. In the case of the coronavirus, PCR tests look for genetic material inside. Using special particles that light up when exposed to the virus’s genetic material, the PCR test focuses only on the virus’s genetic material and ignores your genetic material.  

PCR is an excellent diagnostic tool not only because it can identify viral genetic material, but also because it can make multiple copies of it. So, even if you only have a small amount of viral material in your system, PCR can amplify it to make it easier to find. 

What do results from PCR tests mean?

If your PCR test comes back positive, it means the test found viral genetic material in your system. A person with COVID-19 is infectious and can pass the virus onto others as long as three days before symptoms start (He, 2020). If you test positive for COVID-19 with a PCR test, be sure to quarantine yourself and inform a healthcare provider, as well as any close contacts you had over the few days prior.

If you’ve tested positive in the past, a second positive PCR result doesn’t always mean you’re still infectious. Most people with mild to moderate COVID-19 symptoms typically stop being infectious 10 days after their symptoms start, but the PCR test can remain positive for up to 90 days—even though you don’t have an active infection (CDC, 2020). This is why PCR tests aren’t used to decide if you’re still infectious or if you have to keep quarantining. 

Okay, what if my result comes back negative? Technically, a negative test means no virus was detected, which should mean you don’t have COVID-19. If you’re symptom-free with no known exposure to someone who tested positive for COVID-19, a negative test likely means you don’t have it (Watson, 2020). If you were exposed, however, a negative test result might mean that it’s just too early in the infection to detect the virus.

How accurate is PCR testing?

PCR tests are currently the recommended test for diagnosing COVID-19 because it’s very sensitive (La Marca, 2020). The PCR test does a great job of identifying people who have COVID-19—if you have the infection, the test will most likely be positive. However, no test is 100% accurate all of the time. 

Taking into account user error and normal real-life conditions, PCR tests are about 80% sensitive. In other words, if you tested 100 people with COVID-19, approximately 80 of them would test positive (Yohe, n.d.). That means around 20% of the time, the PCR test may miss people who actually have COVID-19. To counteract this, most healthcare providers will use other information—like potential exposure, symptoms, and high-risk status—when interpreting a negative test.

In summary, PCR tests are great for identifying who has COVID-19, but not as good at telling if you don’t have it. If you have symptoms of COVID-19, especially if you know that you’ve been exposed to a person who tested positive, it’s important to quarantine. Even if your test is negative, do what you can to stay at home and isolate yourself from people within your own household whenever possible (CDC, 2021). 

How long does it take to get results from a PCR test?

Sample collection for a COVID-19 PCR test is pretty straightforward. It can be collected with a cotton swab in your nose (most common) or mouth (less reliable). In some cases, you may be asked to spit in a tube. But no matter what, there are no needles involved. Your sample can be collected anywhere, like at home, in your car, and at the pharmacy. And while it takes just a minute to collect the sample, PCR tests need to be sent to a laboratory or facility with special equipment needed to process the samples. Depending on the facility, it may take hours or even days to get results. 

What is COVID-19 antigen testing?

Like PCR testing, antigen tests are diagnostic and can help determine if you have COVID-19. While PCRs look for viral genetic material, antigen tests look for parts of the virus’s outer shell to see if it’s present in your system.

How does antigen testing work?

Antigen tests specifically look for “spike proteins” on the outer shell of the SARS-CoV-2 virus, which causes COVID-19. Your body does not make these proteins unless the virus is present. By looking for the virus’s outer shell, these tests can tell whether or not you had the virus in your body at the time of the test. 

Antigen testing is kind of like a pregnancy test. You collect the sample (usually from a nasal swab), which is then dipped into a container with a special liquid. The liquid is then poured onto the test strip. After waiting a certain amount of time (usually minutes), you’ll get a yes or no answer. A positive result means there are viral antigens present. That being said, it doesn’t indicate anything other than the fact that there was viral protein present in the sample when you took the test.

What do antigen test results mean?

A positive test means that you have the virus in your body and should quarantine. Avoid leaving your home and isolate away from other members of your household whenever possible. A negative result may be less straightforward. A negative antigen test means that the viral protein wasn’t detected in your sample (CDC,2020). 

If your sample was collected correctly, you have no symptoms, and you haven’t been recently exposed to someone with COVID-19, then it’s unlikely you have it. However, if you have symptoms and still get a negative test result, the Centers for Disease Control and Prevention (CDC) recommends that you take a PCR test to get confirmation that you are truly negative (CDC,2020). 

How accurate is antigen testing?

The great news is there are some COVID-19 antigen at-home testing kits, which means you can test yourself in the comfort of your own home. Some tests require a prescription. Others can be collected at home, but need to be analyzed in a lab. No matter which test you take, you should be aware of any potential pitfalls when selecting one. It’s also important to make sure the test you’re using has received authorization from the U.S. Food and Drug Administration (FDA). 

Antigen tests are typically less sensitive than PCR tests, but sometimes they are sufficiently sensitive to identify cases that are contagious. And that’s the most important thing when it comes to coronavirus tests. 

If you get a negative result and still have symptoms, you might need further testing. The CDC currently recommends antigen tests that are inconclusive or appear negative in people who have symptoms of COVID-19 be confirmed with a PCR test. To keep others safe, you should remain in quarantine while awaiting your results (CDC, 2020).  

How long does it take to get antigen test results?

Antigen testing is inexpensive and fast, especially compared to PCR tests. At-home antigen tests can take as little as 15 minutes to give you results. Some tests can even give you results via your smartphone (FDA, 2020). 

What is COVID-19 antibody testing?

Antibody (or serology) testing is different from the other tests we’ve looked at so far. You can’t use it to diagnose COVID-19 because it doesn’t test for the virus itself. Instead, antibody tests look at your body’s response to the virus. 

When you’re exposed to the virus, your immune system reacts by creating antibodies. As we mentioned, antibodies are specific to each individual organism. They help the immune system to fight off infection and will do so more efficiently the next time you come into contact with that particular bug. In the case of COVID-19, your body forms antibodies against the virus’s crown-like spike proteins. 

Testing for antibodies will only reveal if you’ve been exposed to the virus in the past. It says nothing about your current COVID-19 status. Your body usually starts making the antibodies this test detects around 1-3 weeks after the start of an infection (CDC, 2020). Antibody tests can be especially helpful in identifying people who’ve had COVID-19, but didn’t develop any symptoms and as a result never got tested (CDC, 2021).

How does antibody testing work?

Unlike PCR or antigen tests, these tests look for antibodies to the COVID-19 virus in your blood. The good news is, you won’t necessarily need to go to a lab to get blood drawn (although you may have to if your healthcare provider wants specific testing). Most at-home or in-office COVID-19 antibody tests use only a few drops of blood, usually from pricking your finger with a small needle. Your blood sample is then examined usually with what’s called lateral flow assay technology, which we’ll delve into below. 

To break it down, lateral flow assays are easy-to-use tests that detect the presence or absence of a particle. These tests can be used to detect both antigens (like the virus’s outer shell) and antibodies (your body’s reaction to the virus). In the COVID-19 antibody test, a blood sample is placed at one end of the testing cartridge. As the fluid moves through the cartridge, any antibodies present bind to a special chemical, which creates a colored line. If you don’t have antibodies, then there is nothing to bind to the chemical, and no line appears (Koczula, 2016). 

What do antibody testing results mean?

A negative antibody test means that you likely haven’t been infected with the virus in the past. A positive test means you’ve been infected with the virus in the past. However, it doesn’t mean that you’re currently infected, and can’t be used to determine if you need to quarantine further (CDC, 2020). Unfortunately, we don’t know how much protection these antibodies offer against future infections. That means you should still wear a mask and practice social distancing, even if you’ve had a positive antibody test. 

How long does it take to get antibody results?

The time to get results from antibody testing will depend on whether your blood sample needs to be sent to a laboratory, or is performed on the spot. Most at-home tests will give you results within 30 minutes or less. If your sample has to be sent out, your results will take longer.

Which COVID-19 test should I get?

If you want to know if you’re currently infected with the virus, you’ll need either a PCR or antigen test. 

The next questions to ask yourself are how quickly you need the results, and how certain you need those results to be. PCR tests are more accurate than antigen tests, but it can take longer to get results. 

Type of COVID-19 test PCR  Antigen Antibody
What does it test for? Presence of viral genetic material (diagnostic) Presence of viral proteins (diagnostic) Presence of antibodies to COVID-19
What does a positive result most likely mean? You have the virus  You have the virus You were infected with the virus in the past and recovered
What does a negative result most likely mean? Test didn’t pick up any viral genetic material Test didn’t pick up any viral proteins. Depending on other factors, you may need results confirmed by a PCR test You were not infected with the virus in the past
How fast can you get results? Hours to days Minutes Minutes
What kind of sample? Nasal swab or spit Nasal swab Blood sample
Accuracy Very accurate (recommended for diagnosis) Accurate, but results may need to be confirmed Relatively accurate

Who should get tested for COVID-19?

According to the CDC, you should get tested for COVID-19 if any of the following apply to you (CDC, 2020):

  • You have symptoms of COVID-19.
  • You’ve had close contact with someone confirmed to have COVID-19 (close contact is defined as exposure of 15 minutes or more within six feet of an infected person).
  • You’ve been asked by a healthcare provider, local health department, or employer to get tested.