Your COVID-19 Testing Questions, Answered

Roche experts offer answers to questions about PCR tests, antigen tests, false positives, serial testing and more.

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November 18, 2021

Still trying to figure out the different types of COVID-19 tests and how they work? Roche experts offer answers to questions about PCR tests, antigen tests, false positives, serial testing and more.


How do COVID-19 PCR tests work?


PCR tests for COVID-19 copy, or amplify, bits of the genetic code of the SARS-CoV-2 virus from swabbed samples and detect whether or not the virus is present. Negative results could either mean samples didn’t contain any virus or that there was too little viral genetic material in the sample to be detected. 


What’s great about PCR tests?


  • Accurate and trusted. PCR is the most well-developed molecular testing technique to date. Invented in 1983, it has earned the trust of the medical and diagnostic community. Because they have such high sensitivity, PCR tests are considered the most accurate option available. 

  • Multiplex-friendly. A multiplex test targets more than one virus at the same time. Several types of PCR assays can test for both coronavirus and influenza with one sample, saving time and resources. 

  • Flexible. High-throughput testing on machines in labs provides quick, high-volume results. For example, Roche’s cobas® 8800 can produce up to 1,056 results in an eight-hour shift. If a result is needed in minutes at the point of care, PCR tests like the cobas® Liat come into play. 

  • Variant-ready. Because coronaviruses have a high mutation rate, many PCR tests, including Roche PCR teststarget two regions on the SARS-CoV-2 genome that are unlikely to change, Deeter said. Rapid antigen tests target specific proteins on the surface of the virus that can change.

  • Efficient. Because PCR tests are so sensitive, you don’t have to take them repeatedly to confirm a negative result.


How do COVID-19 antigen tests work?


Antigen tests detect specific proteins on the surface of the virus known as antigens. After a sample is collected by a swab, it’s mixed with a solution that breaks the virus open and frees specific viral proteins. The mix is added to a paper strip that contains an antibody tailored to bind to the proteins, if they’re present in the solution. A positive test result can be detected either as a fluorescent glow or as a dark band on the paper strip.  


What’s great about antigen tests?


  • Spot active infections. Antigen tests can often identify people who are at the peak of infection, when virus levels in the body are likely to be high. 

  • Give fast results. Antigen tests can give results in less than 30 minutes, and the fast results mean that people who test positive can be isolated quickly, before they risk infecting others. Testing repeatedly, known as serial testing, can improve the test’s effectiveness.

  • Cheap to produce and administer. Antigen tests don’t have to be processed in a lab and are inexpensive to produce. Because they don’t require equipment and chemicals, they can easily be used at home, at companies or in schools. They don’t require trained specialists, making them cheaper to administer. 

  • Scalable. When used as a screening tool to frequently assess as many people as possible, rapid antigen tests could be a game changer, according to some experts.

  • Easy to manufacture. Antigen tests are easy to manufacture in bulk 


What about antibody tests for COVID-19?


Antibodies can take several weeks to develop after an infection and often stay in the blood for months to years after recovery, so antibody tests should not be used for diagnosing active infections, Kozak said.


Where can I find and take these COVID-19 tests?


Both PCR and antigen tests are administered at pharmacies, public health departments, hospitals, medical offices and clinics. Consider calling ahead to find out which types of tests are available before you sign up online. You can find take-home antigen tests at pharmacies or order them online. You can also order a kit to take a PCR test at home and send in your sample to be processed by a lab. 


What is viral load?


Viral load or viral level is the amount of virus in your body. “With COVID-19, viral load peaks early, then gradually declines,” Kozak said. “Small amounts of virus may stay in the nose or throat for weeks, months or longer.” There’s a good chance that people with a high viral load are contagious. But determining transmissibility of the virus by looking at viral load is not an exact science. The threshold to tell whether someone is contagious or not still isn’t clear.


What is serial testing?


Serial testing is antigen testing done frequently, multiple times per week (used in congregate living situations, schools and workplaces), and can quickly identify someone with a SARS-CoV-2 infection. “Modeling evidence has shown that frequency and rapid turnaround times offered by serial testing offset the sensitivity issues associated with rapid antigen testing methods,” Kozak said.  Data shared on the CDC website show that serial antigen testing (two or three times per week) is more than 95% effective at identifying infected individuals. The CDC offers guidance on serial testing for workplaces.


Do vaccinated people need to take COVID-19 tests?


Yes. The CDC suggests that people who are fully vaccinated get tested three to five days after coming into close contact with someone with COVID-19. If symptoms develop, it’s time to isolate and get tested immediately. Vaccination can turn COVID-19 into a mild illness, but it doesn’t protect everyone the same way. People in their 70s and 80s and those with serious underlying medical conditions are especially vulnerable to serious illness, even if they’re vaccinated.


Why is a PCR test for COVID-19 more expensive? 


PCR tests are more expensive because processing them requires chemicals, machinery and lab technicians. Antigen tests don’t. But if you’re serial-testing using antigen tests, the cost can add up.

“If you’ve been exposed to someone with COVID-19, you need to test frequently if you’re using an antigen test,” Deeter said. “In your body, the virus is replicating, and at a certain point, there’s enough virus for an antigen test to detect it. By testing over a certain number of days, you catch that peak. On the other hand, because a PCR test is more sensitive, you don’t have to test every single day. So instead of taking five antigen tests, you could take one PCR test that could identify a positive on day one versus five or six days after you’ve been exposed.” 

Some local health departments and pharmacies offer free COVID testing for residents – both PCR and antigen tests – even if they’re uninsured. To get details, contact your health care provider or your state or local public health department. 


Have more questions about COVID-19 testing? Find additional answers.

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Allyson Kozak, Ph.D., MBA, NRCC, is a board-certified clinical chemist and specializes in clinical chemistry, toxicology and immunoassays, including SARS-CoV-2 antibody diagnostics at Roche Diagnostics. She has also served as a laboratory medical director, toxicology expert witness and assistant professor at Case Western Reserve University School of Medicine.


Jamie P. Deeter, MS, Ph.D., obtained her doctorate in infectious diseases, specifically studying viral evolution related to coronaviruses. Previously she has been an ORISE fellow at the CDC, served as a director of research for a small biotech firm and joined Roche’s medical and scientific affairs team four years ago. During the pandemic she has helped launch three of Roche’s SARS-CoV-2 diagnostic tests and has served as a subject matter expert.