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

Home-based rapid tests (also called antigen tests) do not give accurate negative results. However, there are circumstances where they can be a useful tool in helping you avoid infecting people around you and spreading COVID-19 to high risk people who may become severely ill or die.

You just need to know how to use them and understand their limitations.

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

On this page:

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, so it takes a while for you to produce enough antibodies for a positive result.

Unlike the highly-sensitive PCR (polymerase chain reaction) tests performed at Cheshire Medical Center, rapid antigen tests can only detect when there are significant amounts of active virus in your sample.

It is essential to follow the instructions on the box accurately! 

Even though negative results are often inaccurate, antigen tests are helpful because a positive result is usually available within 15 minutes. Also, they are cheap enough to use multiple tests over a few days.

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. Positive results are almost always accurate, early negative results are often wrong. That's 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 give a 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, but a negative test cannot,” says Dr. Khole. “Negative 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.”

Positive results are likely accurate

Positive results: If your test displays a positive result, you can be fairly confident you do have COVID-19 and you are very 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.

Grey area: negative results may NOT be accurate if you have symptoms or have been exposed ti COVID-19

  • Most often, a negative result means it is 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: Stay at home for at least 24 hours and test again 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! Please mask up and sanitize your hands often.

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 already feeling ill—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.

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

One negative result is not trustworthy. If you have symptoms, don't trust a negative result until you get another negative result tomorrow or the next day.

What to do:Stay quarantined, schedule a PCR test, and use the second test again tomorrow when the virus has had more time to replicate. 

Dr. Khole says antigen tests often don’t detect the virus early on.

“It depends on the phase of illness and amount of virus,” he says. “It all comes down to timing."

“At-home tests often don't detect COVID in the first few days. If you have COVID, 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 after a negative test.”

In series

Since negative results from at-home tests are often inaccurate, take a few tests days apart. Most are designed to be used this way with at least 2 tests per package.

Serial self-testing is when a person tests themselves multiple times, or routinely every few days. By testing more frequently, you have a better chance to detect the virus that causes COVID-19 more quickly.

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.

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.

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.