COVID self-test FAQs: Are there false negatives? What does a faint line mean? : Goats and Soda – NPR

The FDA recommends to take at least two COVID-19 antigen tests 48 hours apart before ruling out a possible infection. Justin Paget/Getty Images hide caption

The FDA recommends to take at least two COVID-19 antigen tests 48 hours apart before ruling out a possible infection.

We regularly answer frequently asked questions about life during the coronavirus crisis. If you have a question you'd like us to consider for a future post, email us at goatsandsoda@npr.org with the subject line: "Weekly Coronavirus Questions." See an archive of our FAQs here.

Ah, the start of a new school year. Maybe you're one of millions of Americans who have started mingling with peers in the dorms and suddenly find yourself sniffling and wondering if you have COVID-19.

Or you're just getting back from your summer vacation and the back of your throat has a worrisome itch.

You consider taking an at-home rapid test, but you have lots of questions. With new FDA recommendations on testing, how many times should you test for a definitive result? And, how infectious are you if the positive line is faint? And what if the test turns positive but only after an hour?

We posed your questions to the experts: Dr. Abraar Karan, infectious disease researcher at Stanford; Meriem Bekliz, virologist at the University of Geneva; and Dr. Preeti Malani, professor of medicine at the University of Michigan. Here's the scoop:

So I caught COVID-19 and after 10 days I'm still testing positive. But the line on my rapid test is really faint now compared to a week ago. What's the deal? Exactly how contagious am I?

"The faintness or darkness of the line probably has some correlation to degree of infectiousness especially early on [during the infection]," says Karan.

So if the line is faint, that could mean your risk of accidentally passing the virus on to others is low.

"Some people may not be infectious because the tests could be picking up viral debris from a waning infection," says Bekliz.

But don't rip off your mask just yet: There could be other reasons for a faint line.

There is "some room for error" with those rapid tests, Karan says. You're sticking a cotton swab up your nose and hoping to snare some virus. A faint line could mean you've collected less virus this time around. Maybe you swabbed for less time or in only one nostril when your test instructions say to swab both.

"In general, a darker line is a result of more virus [on the swab]," says Malani. "But antigen tests are not especially sensitive, so even with a negative test, you can be contagious."

So the bottom line, say our experts: If you're testing positive even with a faint line you should behave as if you are contagious.

If there's any hint of a positive line, Bekliz recommends you continue to wear a mask, work from home if possible and generally limit contact with other people.

Last week I tested positive for COVID-19 and went through the recommended 5 days of isolation and additional 5 days of masking. How many times do I need to test negative before it's OK to assume I'm virus-free?

Figuring out when it's OK to ease up on your safety measures after catching COVID-19 can be tricky.

If you suspect that you may have COVID but haven't tested positive yet, the FDA now recommends serial testing, which means you should repeat your home test after 48 hours to make sure you aren't accidentally getting a false negative test.

And what should you do if you already tested positive and want to know when you're finally negative?

"The recommendations for serial testing are for people that have been exposed and are trying to diagnose themselves," says Karan. "They're saying to repeat the test after a day or two, but not if your first test is already positive and you're testing to become negative."

Once you have tested positive and your body starts clearing the virus, a negative rapid test should be a pretty good indication you're no longer infectious, so long as you follow the test protocols correctly and wait an appropriate amount of time before testing.

"If it's been like five days [since the onset of symptoms], and the test is negative, then I would feel good about that," says Karan. "If the test is negative one or two days after symptoms start, then I would maybe think you didn't get a good sample and should test again."

While that's reassuring, there wasn't agreement among our experts about exactly what to do. While Karan says one negative test after an appropriate amount of time is good enough, Malani says you should take two just to be sure.

"Usually you want a couple negative tests," says Malani. "If you have two negative antigen tests, that's really helpful."

So that first negative test is likely a good sign, but taking an additional test 24 hours later is a great way to confirm the result and rule out errors in testing.

I was feeling a bit off and took a COVID-19 test. After 15 minutes it looked like the test was negative. But when I came back to check the test an hour later a positive line had appeared! Then I retested three times and they all came back negative. Does this mean I could have COVID-19?

This is a pretty unusual circumstance, our experts say, but one that could potentially indicate bad news.

"The tests aren't really meant to be read an hour later," says Malani. "But it would be a little bit concerning for someone who has symptoms and has their test turn positive later."

Each test kit has its own recommended timeframe for reading the test. BinaxNOW and iHealth kits say you should read the test after 15 minutes, but not after 30 minutes. INDICAID test kits say you should read the test after 20 minutes, but not after 25 minutes. Reading the test after the recommended time frame could lead to a false positive.

However, Karan agrees that "it would be less likely to be a false positive if someone's having symptoms and we're at a time when there's a high amount of virus being spread."

Following the FDA recommendation to test at least twice and waiting 48 hours between tests can help determine your true COVID-19 status as well.

"My advice would be to probably stay put and not expose anyone, and then perhaps repeat the test," Malani says. "Now, if you're feeling great the next day and your test is negative, maybe it was nothing."

Or simply "assume you're positive if you have symptoms," Karan says. "Try to get a PCR test and retest again [at home] in the next few days. If the PCR test is negative, you're probably OK."

No matter the circumstances, if you're worried that you could have COVID-19, our experts think your best bet is to repeat your at-home test over a couple of days. So if you want to ace COVID Testing 101 this school year, just remember that two negative tests are better than one.

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COVID self-test FAQs: Are there false negatives? What does a faint line mean? : Goats and Soda - NPR