With Covid on the rise once again, you may find yourself reaching for an at-home test for the first time in months. 

Covid activity is picking up, with hospitalizations increasing in recent weeks, wastewater tracking picking up the virus in samples, and a good chance someone you know having tested positive recently. 

Here’s a refresher on how and when to test for Covid, and what your at-home results really mean. 

Do rapid tests work on the variants in circulation? 

The short answer is yes. The at-home Covid tests on the market should pick up all of the variants in circulation, including EG.5, FL.1.5.1 and BA.2.86, said Dr. Michael Mina, a Covid testing expert and former professor of epidemiology and of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health.

“We’ve actually never seen a deterioration in accuracy of the tests, given any new variants,” he said. That’s because the mutations are happening in a different part of the virus than the portion detected by rapid tests. 

“Almost every rapid test that’s authorized in the U.S. and globally detects a protein called the nucleocapsid, which is stable and just doesn’t, hasn’t seen a lot of mutations,” he said. The changes that lead to new variants tend to occur in the spike protein.

Does a positive rapid test mean I’m contagious?

If you get a positive result on a rapid test, it’s most likely you are contagious and can spread the virus. 

“If you have enough virus to turn a rapid antigen test positive, you have a lot of virus there,” Mina said. If the test results show up very quickly and the line is very dark, it could indicate you have a great chance of transmitting the virus to others, he added. 

“You should really assume that you are highly infectious, and do everything you can under the sun to not infect other people,” he said. “But if you’ve gone from having a really dark line to a much fainter line, then you should know that you’re likely still able to transmit the virus, but the potential for that transmission is less.”

There are “extremely rare circumstances” where there are false positives, but the research indicates that a positive test result means you are most likely to be infectious, said Nam Tran, an associate clinical professor in the department of pathology and laboratory medicine who oversees Covid testing efforts at UC Davis Health in California. 

“So I would be cautious if you’re positive, keep the mask on, and avoid contact as we have in the past,” he said.When should I test if I was exposed to Covid?

If you were exposed to someone with Covid, Mina said you are unlikely to test positive in the first day or so. He recommends testing at two to three days after exposure, and if you’re negative, test again the next day. 

The Centers for Disease Control and Prevention still recommends testing at day five after exposure, and Mina said that some people won’t test positive until days six or seven — and that’s within the range of what’s normal.

“But I would say the best rule of thumb is to start testing three days after exposure, and if you’re negative, test again on day four or five,” he said. 

If I test negative, am I in the clear?

If you test negative, many of the same recommendations still hold — you may need to do repeat testing to ensure that the result is accurate. 

“You may have tested too early, and the virus still hasn’t replicated enough yet,” Tran said. 

If you are taking a rapid test at home to see if you have Covid, “it’s usually recommended to test over a few days, once a day, for a couple of days,” he said. 

A PCR test at a doctor’s office or laboratory has a better chance of picking up low concentrations of the virus, so a negative PCR test might be more reliable. “But again, there’s no such thing as a perfect test,” Tran said. “So if you still suspect then you might want to check it again. It’s better safe than sorry, right?”

“If you’re feeling symptomatic, and you think you were exposed, then I would say test again 12 to 24 hours later,” Mina said.

It’s also important to collect the sample correctly, so Tran recommends that you follow the package directions on at-home tests carefully to get the most accurate results.

When should I get a PCR test instead of using an at-home test?

Both tests are good. Tran said home tests had a bit of a bad reputation in 2020 but have become the go-to method for most people when it comes to diagnosing Covid. 

At-home tests tend to be less expensive, faster and easy to use, Tran said, although PCR tests are more sensitive and can still detect lower levels of the virus. Most at-home tests are antigen tests, although a handful of pricier over-the-counter tests are molecular tests, which are more sensitive in a similar way to PCR. “They may not be 100% comparable to the PCR tests run by hospital labs, but they’re nice to have, but expensive,” he said.

Mina doesn’t see a huge advantage in PCR tests, although doctors might want to still use them. “I can’t remember the last time I personally got a PCR test,” he said.” Instead, I just opt for saying, ‘Look, I’m going to have a few rapid tests lying around my house for when I need them.’”

Can I use an expired Covid test?

It depends. In general, experts recommend you don’t use expired tests, but there are some caveats. Some manufacturers have submitted results to the Food and Drug Administration that show that their test kits are still accurate past the expiration date, and people can look up whether their tests are included.

Mina said he also recommends looking at the manufacture date and said that rapid antigen tests may remain reliable up to two years after that date. “Generally they’re all very, very similar, and they’re not going to expire within two years of manufacturing,” he said.

However, if the control line included in the test doesn’t show up or is very faint, it’s not reliable and he recommends throwing the test out.

Tran said it’s not a good idea to use any expired test unless the FDA has confirmed that the results are accurate for that particular test. “We don’t try to see an expired carton of milk and decide to drink it, right?” 

The compounds and chemicals in the test kits can degrade over time, he noted. “I know it costs money and I know it saves time, but if you’re going to get a wrong result that could lead to unintended consequences,” he said.

If you do test positive for Covid, how do you know when you’re negative again?

Although some things have changed since the beginning of the pandemic, if you’ve tested positive and you’re currently positive, you should isolate. According to the CDC, a person should isolate for five days after testing positive, and then wear a mask for another five days or until you have two negative tests 48 hours apart. 

Mina said it’s OK to use antigen tests to make informed decisions based on your family and the health of the people around you. 

“If you’ve gone from a really dark line to a really faint or negative line in three days, you could probably exit isolation, because your immune system probably cleared the virus,” Mina said. 

“You don’t want to infect other people, but you also don’t want to stay in isolation longer than you need to,” he added. “And so I think the best approach is to try to get your hands on some tests and use them judiciously.” 

Why should I even bother to test for Covid?

If you have the sniffles, you may be tempted to assume it’s “just a cold” and not test for Covid at all. While more people have some immunity due to vaccination and previous exposure to the virus, experts say it’s still a good idea to take a test. 

“Despite boosters and vaccines and treatment, transmission still exists and these viruses can break through previously acquired immunity,” said Mina, the Covid testing expert. “Testing remains a mainstay of our collective efforts to limit our potential to infect others.”

People shouldn’t be ashamed to test themselves if they think that they might be positive or if they’re just visiting someone, he said. “It’s just good public health practice,” Mina said. 

And unlike in 2020, there are now more treatments for Covid. 

The antiviral Paxlovid, for example, can be prescribed within five days of a person’s symptoms starting. The drug has been shown to reduce a person’s risk of hospitalization and is being studied to see whether it can reduce the risk of long Covid.

A positive Covid test can help ensure you get the best care you can, Mina said.  

Follow NBC HEALTH on Twitter & Facebook.