With coronavirus cases once more on the rise, testing lines have stretched and store shelves have emptied. People are now turning to at-home testing to avoid the long wait times for results. This new dependence on these tests and their results raises questions. Paul Herrmann, MD, Director of the Clinical Lab at Loma Linda University Health, answers common questions and provides guidance on at-home tests.

What are at-home tests?

At-home COVID-19 tests are rapid antigen tests. They identify proteins, called antigens, on the surface of the virus, and can be processed at home. Antigen tests usually will recognize and identify an active coronavirus case in people with symptoms.

Many rapid antigen tests available without a prescription produce results in about 15 minutes. The test prices start at around $7 each, and the entire process can be completed at home.

“The rapid test works when an antibody—a molecule with a sticky spot that only sticks to a particular antigen protein—adheres to one of the antigen proteins on the surface of the virus,” says Herrmann. “This explains why the test works when there is plenty of virus present.”

However, an antigen test isn’t always accurate. When there is a lesser amount of virus present, the antibody may not find it. In this situation, the test can show a negative result even though there is a virus present, known as a false negative.

On rare occasions, the antibody molecule within the test will even stick to a protein not on the virus, falsely indicating that the virus is present when it is not, known as a false positive result.

Polymerase chain reaction (PCR) based molecular tests, which amplify the amount of detectable material present in a test swab, are more accurate than antigen tests. Because the PCR test makes many copies of the virus’ genes, it detects even tiny amounts of the virus. The sticky interaction between the PCR test system and the viral genetic material almost never identifies a molecule other than the target gene within the virus, meaning a PCR test almost never identifies the virus if it is not present.

Are these tests accurate?

The design reveals a key difference between the antigen and molecular tests. According to Herrmann, the rapid antigen test design can accurately identify the presence of the virus when plenty of the virus is present, but it will not always detect lower levels of it. Because the antigen tests do not always detect the virus when it is present, antigen testing should never be used to rule out a COVID-19 infection.

“The antigen test is not useful if you’re trying to rule out disease,” said Dr. Herrmann. “Negative antigen tests only give a false sense of security.”

Most Omicron cases were contagious for several days before being detected by the rapid antigen tests, according to a recent study comparing PCR and rapid antigen tests. While antigen tests perform best in symptomatic individuals tested within 2-4 days of symptom onset, a one-time result represents only a blip in time. Without symptoms, the test has shown mixed results. A new study found that without a high viral load, 35% of tests produced a false negative result.

“If you have symptoms, even if you get a negative antigen test, you shouldn’t go back to work and think everything’s great,” said Herrmann. “A person who’s sniffling, sneezing, and coughing ought to isolate regardless of the antigen test result.”

Antigen testing can be informative in situations of high disease incidence and symptom prevalence but should still be followed up with a PCR or equivalent test for confirmation.

“If you can get your hands on a good PCR test, there is no reason to do the antigen testing,” said Herrmann.

So now what?

If you develop symptoms or have been exposed to a person sick with COVID-19, immediately self-quarantine and wait.

“No question,” said Herrmann. “Stay home and see what develops.”

Once significant symptoms develop, such as fever, shortness of breath, or loss of taste and/or smell, a positive rapid test result can give a timestamp on when your infection started so that you know how long you need to quarantine.

“The day illness begins can be important in terms of starting the clock for clearing someone to come back to work,” said Herrmann.

Awareness of the prevalence of COVID-19 in your area, your own symptoms and possible exposures provides informative first steps prior to testing.

“People need to pay attention to how they're feeling,” said Herrmann. “There's no substitute for being in tune with one’s symptomatology.”

How do I take an antigen test?

The antigen test involves swabbing the inside of your nostrils and then placing the swab in a few drops of chemicals. It’s a relatively non-invasive process as the swab only collects materials from the mid-nasal region. Every test kit has its own particular process, so following the instructions exactly is crucial.

“The better you follow the instructions, the better the statistics are,” said Herrmann.

What do I do with my result?

If a person receives a positive result, report this information to the public health department as soon as possible. This department uses the data they receive to determine how the virus acts within the population and to make important decisions regarding how best to protect society from the virus.

With a negative result, a positive result, and/or symptoms, Herrmann recommends self-quarantining. If necessary, seek medical attention.

Where can I find an at-home test?

The Food and Drug Administration has approved many rapid antigen tests with Emergency Use Authorization. An increasing number of these tests offer results from the comfort of your own home. Despite the many brands available, the tests function relatively the same way in detection technology and effectiveness.

Due to demand, it may be easiest to order tests online while manufacturers ramp up production to restock shelves. The U.S. government is working to distribute free, at-home COVID-19 tests to its citizens, and tests can now be ordered through a government website.

More information on COVID-19 testing is available online or by contacting your physician.

This article was originally published on the Loma Linda University Health news site.