Do my old at-home tests detect the new COVID-19 variants?

September 12, 2024
Avatar for Jeremy AlfredJeremy Alfred
COVID-19 antigen test kit

Many people use at-home COVID-19 antigen tests when they are sick to keep themselves, their families, and coworkers safe. American households requested over 900 million tests from the COVIDtests.gov program to date. Some kits that people ordered from this and other sources are still unused. As we head towards the cooler fall months, many wonder if some of their older COVID-19 kits still work.

Experts say the tests should work fine, as long as they are not expired.

 

COVID-19 virus structure

Why old antigen tests can still detect new variants

Structurally, the SARS-CoV-2 virus consists of a lipid envelope with viral RNA inside and spikes made of the spike protein protruding outside. The viral RNA is encapsulated and protected by the nucleocapsid protein.

The spike proteins bind with the receptor proteins on host cells. Upon binding, they change their conformation in such a way that the membranes of the virus and the host cell are brought into close contact. Once the membranes have fused seamlessly, the viral RNA enters the cell’s cytoplasm.

The spike proteins are targets of intense immune response and are under constant evolutionary pressure to mutate. To date, all new variants of SARS-CoV-2 are a result of mutations in the spike protein.

Antigen kits are made to detect the nucleocapsid protein. The nucleocapsid protein has not changed enough since the first antigen-detecting kits were made, so older kits can still detect the new variants reliably.

On the other hand, the spike protein has changed enough that immunity from the first round of vaccinations or infections may not target the new strains sufficiently.

Do antigen tests expire?

Antigen tests can expire. The tests are made of many components such as the antibody that binds to the virus, another antibody that ensures the test is working properly, solutions in which the nasal swab is washed in, etc. Components can degrade over time resulting in unreliable results from the expired test. Therefore, the FDA does not recommend using at-home antigen tests past their expiration date.

However, the expiration date on the box may not always be accurate. Some kit manufacturers may have extended their kit’s expiration date. If so, the most updated information about whether a kit’s expiration date has been extended can be found on the FDA’s website.

When to take an at-home antigen test?

If you are sick with any COVID-19 symptoms or have been in contact with individuals who may have COVID-19 infection, you can take an antigen test to learn if you have COVID-19. At-home antigen kits accurately detect the virus at least 80% of the time.

The CDC recommends testing immediately if you have symptoms. If the test is negative, test again after 48 hours, or take a PCR-based test. A PCR test is more accurate at detecting COVID-19 infection.

If you are asymptomatic, wait 5 full days after your exposure to the SARS-CoV-2 virus. If the test is negative, repeat the test after 48 hours. If you test negative again, test a third time after 48 hours.

What if I keep testing positive weeks after I am better?

You can test positive for about 10 days after getting sick as there is still enough virus in your body for the antigen test to detect. More sensitive PCR kits can detect the virus up to 3 months after an infection. If you recovered from COVID weeks ago and still test positive on the antigen kit, it may still be from the past infection. But pay attention to your symptoms because you may have a COVID-19 rebound or reinfection. Usually, our bodies have immunity after COVID-19 infection for up to 3 months after recovery, but reinfections are still possible. The CDC advises patients to see their medical provider so they can closely assess their symptoms, medical history, and overall health to guide them.

The information provided in our blog posts is for informational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this blog.

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