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COVID-19 Testing Explained

COVID-19 Testing Explained

December 23, 2021

As families and friends gear up to celebrate the holiday season, many are taking COVID-19 tests to protect their loved ones. With more infectious breakthrough cases––through variants like the Omicron––increasing worldwide case numbers, regular testing, especially before social gatherings and travel, will play a vital role in lessening the spread of the virus.

With these circumstances in mind, we’re addressing some common questions regarding the types of testing methods, their accuracy, and best practices below.

Types of COVID-19 Tests

What are the types of tests for COVID-19?

The two most common types of tests are rapid antigen tests and polymerase chain reaction (PCR) tests. Rapid tests are often more readily available, providing quicker results (typically within minutes). Meanwhile, PCR tests are more accurate at detecting early infections, but take longer to produce results (hours to days). To receive the most accurate results, it’s important to understand when and how to use each test.

PCR test:

When it detects COVID-19:

While the PCR test identifies infections early up to 2 to 3 days post exposure (as denoted by the dotted line in the graphic below), it also detects COVID-19 after the virus is no longer infectious. In doing this, the PCR test provides “late phase” positivity between days 10 and 15 (shown by the red dot along the post-infectious phase). PCR tests are generally performed outside the home, and tests are sent to a laboratory for processing. Results may take hours to days to return. 

When to use it:

PCR tests should be used instead of rapid antigen tests if it has been less than 4 days since the exposure, and the individual is able to wait a day or two for a result. PCR tests are also more accurate if you have active COVID-19 symptoms.

Rapid antigen test (aka lateral flow test):

When it detects COVID-19:

The rapid antigen test may not detect the virus in an infected individual until 4 days after the initial exposure (presented by the solid line in the graphic below). In contrast to the PCR test, once outside the infectious phase, the rapid test will read as negative. Most of the at-home tests are rapid antigen tests, and results can be available within minutes. 

When to use it:

Rapid antigen tests are most useful when it has been several days since an exposure and a result is needed immediately. Repeating tests on consecutive days may increase the test’s reliability in identifying infection.

Here’s a helpful comparison of the two tests:

If you come in close contact with someone who’s tested positive for COVID-19, you should isolate and test several times. Even if you are contagious, it may take several days for you to test positive or show symptoms. 

Omicron + Testing

Does the Omicron variant change this testing information?

According to the World Health Organization, PCR tests continue to detect the Omicron variant at the same rate as prior variants. While it’s still unclear how accurate rapid antigen testing is for the Omicron variant, initial information from the testing companies indicate that it can still detect the virus. However, independent studies are required to confirm the test’s accuracy and effectiveness. 

Important Reminders:

  1. Continue to wear masks in public.
  2. You can be contagious and still test negative. Both tests require time for the viral load to build before detecting the virus, during which time an individual may still be contagious. The PCR takes about two days to identify a positive case, compared to up to four days for the rapid antigen test. Serial testing over subsequent days may increase the likelihood of obtaining a positive result in an infected individual.
  3. Test often. Perform a rapid antigen test immediately before attending social gatherings without masks.
  4. Avoid contact with others if you have symptoms. If you have symptoms of COVID-19 (cough, fevers, aches, runny nose, sore throat, etc.), individuals should avoid contact with others until they receive a negative test result to avoid the risk of spreading the virus.


Wishing everyone a healthy safe holiday and new year!

Disclaimer:

This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment and does not constitute medical or other professional advice. All calculations provided in this article are using publicly available data and should not be taken as absolute. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Sources:

  1. Mina, M., et al. Rethinking Covid-19 Test Sensitivity — A Strategy for Containment. NEJM, 2020.
  2. https://www.who.int/news/item/28-11-2021-update-on-omicron

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