By Aparna Zalani

DALLAS (CBSDFW.COM) – Public health officials are urging Americans to get tested before and after traveling or attending a family gathering this holiday season, but how accurate are these tests and when should you take them?

At-home COVID-19 tests can be purchased over-the-counter at drug stores and pharmacies.

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“The accuracy depends a little bit on what we call prevalence, so how much virus is around,” said Dr. Trish Perl, Chief of Infectious Disease at Parkland Health and UT Southwestern Hospital.

“The more virus is around, the more accurate these tests can be.”

There at two types of COVID-19 tests; PCR and antigen.

The PCR is the gold standard, often used in clinical setting. It is also more accurate.  Antigen tests are used for rapid tests and in at-home tests kits.

“In general, they are much more accurate if you’re symptomatic than if you’re not symptomatic,” Dr. Perl said about the antigen tests.

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So if you have no symptoms and are trying to get together with family, Dr. Perl said the best thing you can do is to is test multiple times.

She said the at-home tests aren’t perfect. They may not tell you if you are infected, but they could tell you if you are infectious.

“If you do it right before you go to an event and you have a negative test, you can feel relatively comfortable that you’re not infectious,” she said, adding that you should retake the test two or three days later to confirm that you didn’t have the virus incubating at the time in case you need to notify the people who you were with.

If you want to check if you were infected after a gathering. Dr. Perl said to test 26 to 48 hours after the party and then again two or three days later. “If you develop symptoms in the interim, then you want to go in and get one of the PCR tests,” she said.

The PCR tests are more accurate but it takes time to get the results back.

Dr. Perl says if you have had a known exposure to someone with COVID-19, it’s best to test for the virus then quarantine and repeat the test 4-5 days after the exposure. For safety, wear a mask and stay physically distanced.

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The at-home tests typically come in packs of two. Testing multiple times increases the accuracy and also accounts for human error. You can find a list of FDA authorized at-home tests here. Just click the antigen link and the molecular link, and on either page type OTC into the box. That narrows the list to 3 molecular and 11 antigen tests.