When and how should I use home-based, rapid COVID-19 antigen tests?

Home-based rapid tests (also called antigen tests) can be yet another great tool, along with personal protective actions, to prevent the spread of COVID-19 and keep your family and friends out of the hospital this holiday season. But you need to know how to use them and understand their limitations.

Act Now: on 12/23/21 New Hampshire released more free tests for residents to order via sayyescovidhometest.org while supplies last.

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Girl gives a thumbs up as she holds up a negative rapid test.

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What is an antigen test?

These tests are available over-the-counter at pharmacies so you can conveniently test yourself at home. Antigen tests detect antibodies reacting to an active COVID-19 virus.

Unlike the highly-sensitive PCR (polymerase chain reaction) tests performed at Cheshire Medical Center, rapid antigen tests can only detect when significant amounts of active virus are present via a nasal swab or saliva sample.

It is essential to follow the instructions on the box accurately. Only a positive result should be trusted. However, antigen test results are usually available within 15 minutes and many tests are inexpensive enough to use in series for more accurate results. A negative test is never a free pass. You still need to follow precautions to protect yourself and others.

UMass explains how antigen and PCR tests work.

Learn more on the CDC’s Self Test page.

Are at-home COVID tests accurate?

“It depends,” says Aalok Khole, MD, infectious disease specialist at Cheshire Medical Center, “on the probability of an individual having COVID-19, because these tests are specific but not sensitive.”

“Specific” means antigen tests only react to active, replicating virus. “Not sensitive” means antigen tests need a lot of live virus to display an accurate positive result.

“If you have been involved in any activity where there was a likelihood of COVID-19 transmission, a positive antigen test can be trusted,” says Dr. Khole. “If not, antigen test results need to be taken with a pinch of salt, given their low sensitivity. One way to get around that is testing more frequently.”

Likely Accurate

Positive results: If it's likely you may have COVID-19 (such as exposure, attending a gathering without masks, or having symptoms) and your test displays a positive result, you can be confident you do have COVID-19 and you are infectious right now.

What to do: Isolate. Report the positive case to your provider via messaging within myD-H or to New Hampshire Health and Human Services. See NH DHHS requirements for isolation.

Positive without risk: If your home test shows a Positive when you have no symptoms AND no risk of being exposed, it may still be accurate with such high rates of community transmission. However, there is only a very small chance you could get a false positive.

What to do: Assume you are infectious with COVID-19 and isolate, then retest in 24 hours or schedule a more sensitive PCR test via myDH.

This differs from PCR test results because those are so sensitive they can usually detect that someone has COVID-19 a day before symptoms begin and then weeks, or longer, after someone has stopped being infectious.

Grey area—use other measures

A) If you have symptoms or are likely to have COVID-19 but it shows Negative, you may not get an accurate result because:

  • It may be too soon in your infection for the non-sensitive antigen test to detect enough active virus.
  • You may not have picked up enough active virus on the swab.
  • You may not have followed instructions accurately and completely.
  • If it has been longer than 3-5 days since your symptoms began, you may have COVID-19 but are no longer highly infectious so there is not enough active virus in your nose for this test to detect.
  • Any combination of the above.

What to do: Remain quarantined and test again in 24 hours or schedule a more sensitive PCR test via myDH . Even if not COVID-19, be careful not to give anyone whatever virus you do have! Mask up and sanitize your hands often.

B) As a courtesy for others: If you do not have symptoms and are not likely to have COVID-19 (such as a person who has received a booster recently or hasn't been near others) testing before a gathering can still be a good idea to ensure the safety of those you come in close contact with. Use an at-home test 2-3 days before the gathering AND an hour or two before you go.

Unlikely to be accurate

If you don’t have symptoms but you are likely to have COVID-19 due to close contact or being unvaccinated and unmasked in public, one negative antigen test result should NOT be considered accurate.

Antigen tests are rarely sensitive enough to detect a pre-symptomatic infection. People can develop COVID-19 a week or two after exposure and usually become infectious to others a day or two before symptoms appear. Even if your test is negative because the virus hasn’t replicated enough to be detected now, you may become infectious mere hours later.

If you are unvaccinated: stay home and schedule a more sensitive PCR test for 3-5 days after exposure via myDH or home test again in 48-72 hours.

If you are fully vaccinated: continue to wear a mask and take personal protective actions for those around you. Schedule a more sensitive PCR test for 3-5 days after exposure via myDH or home test again in 48-72 hours or just before you go out.

If you are fully vaccinated with a booster: continue to wear a mask and take personal protective actions for those around you. Home test again home test again in 48-72 hours or just before you go out.

When are at-home rapid antigen tests most useful to use?

When you feel sick

Save yourself some hassle by buying some rapid tests now in case you ever develop symptoms.

Driving to get a PCR test is far from appealing when you're feeling ill and you may not receive results for 2 to 4 days, depending on available appointment times during high demand. This is the best time to use an antigen test.

A positive antigen test within the first 5 days of developing symptoms is very likely to be accurate. You can trust that you have COVID-19, and are infectious. Follow isolation guidance and report your positive result to NH DHHS.

When you have symptoms, one negative antigen test should not be trusted, even if you followed the instructions exactly. Stay quarantined, schedule a PCR test, and use the second test again tomorrow when the virus has had more time to replicate. If your second rapid test is positive, you can always cancel your upcoming PCR test appointment for someone else to use.

Dr. Khole says antigen tests often don’t detect the virus early on if you don’t have symptoms.

“It depends on the phase of illness and amount of virus,” he says. “It all comes down to timing, especially in an asymptomatic individual.”

“There are times when COVID may not be detected on the first day you do the test. If you are COVID positive, the amount of virus in your body often increases after a couple days of your initial exposure,” says Dr. Khole. “That is why it’s recommended to take retest 2-3 days later when the viral load can be high enough to cause a positive test.”

In series

Due to the frequency of false negatives given by at-home tests, they are most useful when done in series over a few days. Many are designed to be used this way and come with more than one test in a package.

Serial self-testing is when a person tests themselves multiple times for COVID-19, or on a routine basis, such as every few days. By testing more frequently, you might detect the virus that causes COVID-19 more quickly and could reduce the spread of infection.

You can use at-home tests this way if you want to attend a gathering where people will be unmasked. As long as you don’t have symptoms (stay home if you do!), test 2 to 3 days before the event and again right before you leave. For the reasons listed above, masking, physical distancing, ventilation, and frequent hand sanitation are still important as the Omicron virus variant enters our community.

What should I do if my antigen test shows positive?

Trust that you have COVID-19 and are infectious right now.

Dr. Khole says “If you test positive for COVID, isolate yourself at home, avoid contact with others, and monitor your symptoms. If you feel like your condition is deteriorating, contact your provider.”

“As soon as your isolation period ends, if you are unvaccinated or need your booster, please get vaccinated or boosted. Natural immunity is not as effective as getting vaccinated.” Dr. Khole explains, “getting vaccinated after infection will help boost your immune response further to prevent reinfections, especially when variants are circulating. There is nothing to lose from getting vaccinated and there is a lot to gain."

Follow DHHS isolation guidance and report your positive result to NH DHHS.

How do I get an antigen test?

It’s best to stock up on antigen tests BEFORE you need them because they can be hard to find.

  • Local pharmacies often list on their websites if they have rapid tests in stock, but call them first before you drive to buy one because they often list inaccurate stock counts or disappear quickly.
  • Purchase online from pharmacies, big box stores, or Amazon, but expect 1-3 weeks for delivery due to high demand.

What else should I do?

As always, the effective personal protective measures for yourself and your family are:

  • Wearing masks
  • Getting vaccinated and boosted
  • Sanitizing your hands frequently
  • Avoiding unventilated or crowded spaces
  • Physical distancing
  • Staying home while sick or exposed
  • PCR testing between day 2-4 if asymptomatic
  • Maintaining a strong immune system by exercising daily, eating a nutritious diet, sleeping well, and managing stress.