(CNN) — If you think a negative test result means you don’t have coronavirus, you may be wrong.

It can take days for a new infection to show up on a Covid-19 test.

“We know the incubation period for Covid-19 can be up to 14 days. And before that, you can test negative and have no symptoms,” said emergency physician Dr. Leana Wen.

“But you might actually harbor the virus and be able to transmit it to others.”

So if you want to get tested before seeing friends or family, here’s what you need to know:

If I was infected yesterday, would a test today pick it up?

Probably not. A study in the medical journal Annals of Internal Medicine reviewed false negative test results from people who actually had Covid-19.

The study estimated that for four days of infection before symptoms have usually started, the likelihood of getting an incorrect/negative test result on day 1 was 100%.

On the day people started showing symptoms, the average false-negative rate had dropped to 38%, according to the study. Three days after the onset of symptoms, the false negative rate dropped to 20%.

“The virus just takes time to replicate in the body at detectable levels,” said Justin Lessler, lead study author and associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

“You may be infected with just a few virus particles, but those won’t be detectable until they have time to replicate to adequate levels to be detected,” he told CNN via E-mail.

So how many days should a person wait after possible exposure to get tested?

“There’s no hard and fast rule, but the evidence suggests getting tested before the third day after exposure isn’t very helpful,” Lessler said.

Could I be contagious when the test is negative?

Absoutely. “People feel like if you test (negative) you’re off the hook. And you’re kind of not,” said Dr. Rochelle Walensky, chief of the division of infectious diseases at Massachusetts General Hospital.

For people who become ill with Covid-19, symptoms can take up to two weeks to appear, but the average delay is around five days, Walensky said.

“You are generally thought to be most contagious the two days before that day and the two days after,” she said.

One of the reasons this virus spreads so easily is that people can be infectious without any symptoms. The US Centers for Disease Control and Prevention estimates that 40% of infections are asymptomatic and 50% of transmissions occur before symptoms start.

“That’s been one of the biggest Achilles’ heels of that. And that’s been among the biggest challenges and the unexpected things because (with) his cousins, that’s not true,” Walensky said.

“The reason we were able to control the SARS outbreak so quickly – although obviously there were a lot of deaths there – but it didn’t turn into a pandemic, is because people weren’t shedding (the virus) until they had symptoms.”

Are the different Covid-19 tests important?

There are two main types of diagnostic tests that try to detect if you have an active coronavirus infection:

Molecular tests, such as PCR tests, look for the genetic material of the virus. Most of these tests are done with nasal or throat swabs, although some can be done with saliva, the US Food and Drug Administration said.

“This test is usually very accurate and usually does not need to be repeated,” the FDA says.

But the downside of molecular testing is that the results can take a while, from the same day to a week after the test.

“For people with symptoms, studies so far show that the accuracy of molecular testing in finding a positive case increases each day after exposure,” said Pia MacDonald, infectious disease epidemiologist at the non-profit research institute RTI International.

But for infected people who don’t show symptoms, the accuracy rates are less clear, she said. “Performance studies of molecular tests on asymptomatic people are very limited.”

Antigenic tests are often called rapid tests (although some molecular tests are also quick). Antigen tests are not antibody tests, which tell you if you have previously had the virus and have already developed antibodies against the infection.

Antigen tests do not look for the genetic material of the virus, unlike molecular tests. Instead, they look for specific proteins on the surface of the virus.

The good news is that you can get antigen test results in less than an hour. The bad news is that you’re more likely to get a false negative with a rapid antigen test.

“Positive results are usually very specific, but negative results may need to be confirmed with molecular testing,” the FDA says.

“Antigen tests are more likely to miss active coronavirus infection than molecular tests.”

This could help explain some of the recent spread of Covid-19 linked to the White House.

While staff members close to President Donald Trump and Vice President Mike Pence are frequently tested, White House staffers often use rapid antigen testswhich generally have a higher false negative rate than molecular tests.

Regardless of the type of diagnostic test you use, you are generally more likely to get a false negative than a false positive.

“If a molecular test is positive, that’s an accurate reflection of an infected person,” MacDonald said.

“If it’s negative, it’s less reliable that the person is actually negative. The same goes for antigen testing.

Can I test myself at home?

Yes. There are home testing kits available, such as at Everlywell Covid-19 Molecular Test. Users collect their own nasal swab samples and mail them to a lab, which will send the results digitally within 24-48 hours of receiving the samples.

But taking any type of Covid-19 test too soon could miss an infection, said Dr Frank Ong, medical and scientific director of Everlywell.

“As testing capacity has continued to increase, more asymptomatic or mildly symptomatic people have been tested, most of whom likely have lower viral loads in their clinical samples,” he said. .

So what should I do if I want to see friends or relatives?

If you insist on seeing someone who doesn’t live with you, it’s best to self-quarantine for 14 days in advance, Walensky said.

“If you do it right, you don’t need a test,” she said. “That’s probably the cleanest way to do it.”

To be clear: quarantine means staying home. It doesn’t mean you can shop.

“‘Grocery’ and ‘quarantine’ don’t belong in the same phrase,” Walensky said.

Lessler agreed that quarantine is the best course of action and that any testing should be done smartly.

“If you are visiting an elderly family member and you have a reasonable risk of having been exposed, there is no substitute for 14 days of quarantine,” Lessler said.

“At the very least, I would wait 10 days (of quarantine) and test negative,” he said.

“If you are visiting a younger, healthy family member and are unlikely to be exposed before or during the trip, then 5 or 7 days (of quarantine) plus a negative test is probably a significant reduction in risk, but no guarantee of safety.”

What should I do after a negative test?

It is important to strictly quarantine not only before your Covid-19 test, but after your test too.

“You definitely need to stay in quarantine while you wait for test results and make sure everyone you meet with is on the same page about the infection risk control plan,” Lessler said.

There have already been cases of the coronavirus spreading within families just days after someone tested negative, said Dr. Michael St. Louis, a member of the CDC’s community referral team.

He said everyone should remember to treat family from different households the same way you would treat unrelated friends or work colleagues during this pandemic.

What’s the safest way to celebrate Thanksgiving?

The best way to make sure everyone stays healthy is to celebrate the holidays from a distance.

“I have three children. … And my parents won’t be joining us this year,” Walensky said.

“It’s just awful. But what I really take away is that my parents are pretty healthy, and I would never forgive myself if I put them in danger. And I look forward to a year 2021 where we can be together.

The CDC suggests celebrating with loved ones virtually online. You can also make traditional Thanksgiving dishes and deliver them”in a way that does not involve contact” to parents, neighbors or those who might feel alone.

Walensky said the small sacrifices made for Thanksgiving will help ensure everyone is healthy enough to sit down to dinner next year.

“At least they’ll be here next year, when irresponsible behavior now could mean they won’t be here later,” she said.

“Let’s do this so we can have a much better chance of being around the table together, healthy, in 2021.”

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