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Sharp Health News

Which COVID-19 test is best?

Sept. 21, 2020

COVID-19 swab test
COVID-19 testing experiences across the country have been varied. While one person may receive test results within 24 hours, another can wait more than a week to know whether or not they have been infected. Some people choose to pay over $100 out of pocket at a pop-up testing site for a $5, 15-minute test that may or may not be accurate.

Dr. Omid Bakhtar, a board-certified pathologist affiliated with Sharp HealthCare, stresses that it is the type of COVID-19 test being used that truly counts. And different tests may better serve different situations.

Types of COVID-19 diagnostic tests
According to the Centers for Disease Control and Prevention (CDC), testing guidelines have changed throughout the duration of the pandemic and may continue to do so. There are currently two primary types of approved diagnostic COVID-19 tests in use:

  • PCR tests — known as molecular, genetic or RNA tests — detect the virus’s genetic material. The sample is taken with a nasal or throat swab or from a person’s saliva. PCR tests are highly accurate but results can take up to a week or longer. In fact, these tests are so incredibly sensitive that in some scenarios they may detect “genetic” fragments of a prior or waning infection, meaning the person whose sample was tested and is deemed positive may be carrying a very low viral load and is less likely to be contagious.

  • Antigen tests, sometimes referred to as rapid tests, detect specific proteins on the surface of the virus. The sample is taken with a nasal or throat swab and results can be produced within an hour. While positive results are usually accurate, negative results may need to be confirmed with a PCR test, especially if a person is experiencing symptoms of COVID-19. These tests perform best when the person is tested in the early stages of COVID-19 infection when viral load is highest.
Another type of testing, antibody testing — also known as serology testing — screens for past infection. The test determines the presence of antibodies, which are proteins that fight off infections and can often provide immunity against catching the same infection again. However, most currently available antibody tests do not screen for active COVID-19 infection.

“There is a certain level of confusion surrounding testing, even among those in the medical field,” Dr. Bakhtar says. “While antigen tests are faster, less expensive and don’t require the level of laboratory equipment that PCR tests do, they are specifically made to be used in people who are symptomatic, and these tests have a higher likelihood of producing false-negative results.”

According to Dr. Bakhtar, more than 50% of people with active COVID-19 infection are asymptomatic or presymptomatic, so a negative antigen test result should lead to a follow-up PCR test for confirmation. However, this isn’t always being done, and a false-negative antigen test result for an asymptomatic person could lead them to drop their guard and ignore precautions, such as wearing a face covering and social distancing, further spreading the virus.

The role of screening and surveillance testing
“It’s important to note, though, that antigen testing is not without utility,” Dr. Bakhtar says. “Antigen tests are valuable when used to perform screening within large populations, such as students living on a college campus.”

This type of screening must be done repeatedly to quickly identify people with active COVID-19 infection and to accumulate data on the rate of infection within the population. Moreover, repeat testing can make up for some of the limitations seen with antigen testing. Specifically, false-negative tests are more likely to be identified if the test is repeated on the same individual.

Along with diagnostic testing and screening testing, there is also surveillance testing, which is an ongoing and systematic collection of data usually performed by public health agencies. Surveillance testing for COVID-19 is intended to randomly select and sample a percentage of people in a population on a rolling basis to assess local infection rates and trends, rather than identify any one person’s infection. Cumulative results of surveillance testing are returned only to the public health agency or institution, not the individuals tested.

Who should be tested
Not everyone needs to be tested for COVID-19. However, the CDC recommends that you should talk to your doctor or a public health professional if you have:
  • Symptoms of COVID-19
  • Had close contact (within 6 feet of an infected person for at least 15 minutes) with someone who is confirmed to have COVID-19
  • Been asked or referred to get testing by your health care provider or the health department
Once tested, you should quarantine at home until you receive your test results. If you test positive, follow all isolation and prevention guidance to protect others from getting sick. If you test negative, it means that you probably did not have COVID-19 at the precise moment your sample was collected.

However, it’s important to remember the risk of false-negative results if a rapid antigen test was performed, and a follow-up PCR test may be in order, especially if you have been exposed to COVID-19 or are experiencing symptoms. Until that test can be performed, you should continue to follow prevention guidance.

“Knowing whether you have COVID-19 is a ‘here and now’ situation, as a test today can’t tell whether you might have it two, five or 10 days from now,” Dr. Bakhtar says. “If you have been exposed to someone with COVID-19 or are experiencing symptoms, you should get tested, but make sure that it is with the right test that can give you accurate results. And in a case like this, that would generally be a PCR test.”

Learn about COVID-19 screening and testing at Sharp HealthCare hospitals and clinics.

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