In November 2020, the Food and Drug Administration issued an emergency use authorization for the first at-home COVID-19 test, for prescription use only. A month later, the agency issued an EUA for the first over-the-counter rapid antigen test.

Now, the FDA lists 16 approved antigen home tests from 12 manufacturers, including one approved as recently as Dec. 29. With results in 15 to 30 minutes, these tests provide results much faster than polymerase chain reaction, or PCR, tests but they're less sensitive, meaning there's a greater chance of false negative results.

With the surge in coronavirus cases from the omicron variant has come a surge in demand for at-home tests, as people sought to safely see family over the holidays and as kids and teachers have returned to school.

But if you can get your hands on a test, when is the best time to take one and how should you interpret the results? Along with reading studies, we consulted with three people in the region: Dr. Oliver Mayorga, chief medical officer at Lawrence + Memorial Hospital in New London; Steve Mansfield, director of Ledge Light Health District; and Patrick McCormack, director of Uncas Health District.

When is the best time to take a rapid test, and how should you interpret results?

"Generally, the rapid tests that the state is providing and all the rapid tests are really good at determining whether or not someone is COVID-positive," Mansfield said. "They're not as accurate in determining if someone is COVID-negative."

False positives are rare, and Mayorga said if you have symptoms and test positive, you can be pretty sure you have COVID-19.

But "what can happen with rapid tests, they may not pick up enough viral particles to become positive, even though you may be infected and infectious," Mayorga said. That's why the current recommendation if you test negative is to retest yourself 24 to 48 hours later with another rapid test, and why some of the FDA-authorized kits come with two tests.

"If they're positive on the test they should treat it as a positive, and if they're negative, they should retest," McCormack said. Mayorga recommends that people read the manufacturer's instructions and fine print on the tests for more information.

With the increased difficulty of finding PCR tests, Mansfield said rapid tests are especially important for people identified as a close contact of someone with COVID-19.

Let's say you were exposed to someone who tested positive for COVID-19. When should you get tested?

People may find that a rapid test one or two days after exposure comes back negative, whereas a test two days later is positive.

The Centers for Disease Control and Prevention still recommends that people get tested at least five days after close contact with someone who has COVID-19, even if they don't develop symptoms and regardless of vaccination status.

In a FAQ, the Connecticut Department of Public Health said people should use a rapid test "before going to indoor gatherings with people who are not in your household, in order to make sure you are not spreading infection" and "if you are feeling sick to quickly learn if you might have COVID-19."

Mansfield also noted that when it comes to testing requirements for travel or work, results from a rapid test may not be accepted.

How accurate are rapid tests?

A meta-analysis published in the journal Diagnostics on Jan. 4, looked at the sensitivity and specificity of rapid antigen tests from 94 studies in 12 countries published up until the search date of Aug. 26. Sensitivity refers to the ability of a test to correctly identify those who have COVID-19, while specificity refers to the ability to designate a person without the disease as negative.

The meta-analysis showed an overall pooled sensitivity of 70% and specificity of 98%. The sensitivity was 82% for symptomatic patients but 68% for asymptomatic ones, meaning that COVID-positive people without symptoms may test negative on a rapid test.

A total of 74,445 samples from 30 different rapid tests were included in the analysis, but there was a lot of variation among the tests: The sensitivity ranged from 37% to 90%, while the specificity ranged from 65% to 100%.

With omicron, the FDA said Dec. 28 that early data suggested antigen tests do detect the variant but may have reduced sensitivity.

At the same time, Mayorga noted, "Antigen tests have gotten better, much better. There are some that are more than 90% at detecting (COVID-19) infections, if you're symptomatic. If you're not symptomatic, it's still low, but not to the degree where it was literally a coin flip in the very beginning."

DPH said in a clinical study where the iHealth COVID Antigen Rapid Test, which got its EUA on Nov. 5, was compared to a molecular test, it correctly identified 94.3% of positive specimens and 98.1% of negative ones.

Where and how can you get a rapid test?

The Biden Administration announced Friday that starting Wednesday, people can visit covidtests.gov to order at-home tests at no cost. A half-billion tests will be available, with an initial limit of four per household, and the White House said tests typically will ship within seven to 12 days of ordering.

The White House also said it will launch a call line for those who can't access the website, but a number hasn't yet been provided.

To avoid the hassle of running around from store to store, some pharmacies may allow customers to order a rapid test online for in-store pickup. For example, as of Saturday morning, Walmart listed this as an option for the BinaxNOW test.

Those interested in getting a rapid test also can check Target's website to see if one is available at a nearby store. Other pharmacies that may have rapid tests include CVS, Walgreens and Big Y.

As of Saturday, private insurers are required to cover the cost of up to eight home tests per month.

East Lyme had a test distribution scheduled for Saturday but postponed it due to the wind chill. A new date hasn't been set, but town residents can check back on the East Lyme Public Safety Facebook page, town website and local television access channels.

e.moser@theday.com