What does a negative result on a home
COVID-19 test really mean?
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That’s the question that has confounded many people who have
reached for a home test because they have a sore throat, cough or runny nose.
After swabbing their nose and waiting an anxious 15 minutes, the result is
negative.
While there’s relief in getting a negative result, there’s also
uncertainty. Am I really free of COVID? Or did the test just not detect it?
Should I test again? Can I spend time with other people?
The confusion is justified, say testing and public health
experts. It stems from a lack of understanding about how the tests work. Rapid
home antigen tests look for pieces of viral proteins from a swab of your nose,
and they are designed to identify whether you have an infectious level of the
virus. But a negative test is not a guarantee you don’t have COVID.
It could be that your symptoms are an immune response signaling
the arrival of COVID or another invader. The harder your immune system is
working to tamp down the virus, particularly an immune system supercharged by
vaccine antibodies, the more likely you are to get an early negative result on
a rapid test, even if you’re infected.
“It may be that the virus in your body is having a tug of war
with your immune system,” said Dr. Michael Mina, chief science officer for
eMed, a company that helps rapid test users get treatment from home. “If you
test negative and you have symptoms, don’t assume you’re negative. Assume that
the virus has not had an opportunity to grow up yet. The symptoms might mean
your immune system is just triggering a very early warning.”
Mina advises people to take a rapid test on the first day of
symptoms. A positive result means you almost certainly have COVID. If the
result is negative and your symptoms continue, you should still take
precautions, wear a mask and avoid close contact with other people. If you
can’t test daily, then wait 48 hours and test again. If you’re still negative
but your symptoms persist or are getting worse, you should take another test on
Day 4. Or you may want to go to a testing center to take a PCR test, which can
sometimes detect COVID a little sooner than a home test, although you may have
to wait a day or two for the results.
Experts say that if you have symptoms and continue to get negative
results on home tests, it may be that your immune system is doing a good job
beating the virus. Or it could be that you have another illness. Either way,
you should try to avoid infecting others.
“If you have symptoms and continue to test negative, the chances
that you’re infectious with COVID have gone down a lot,” said Dr. Robert
Wachter, chair of the medicine department at the University of California, San
Francisco. “But you probably should wear a mask that day because you have
something.”
And, remember, the result of your home test is just one piece of
information. If you haven’t left the house in weeks, your negative result after
a few tests is probably accurate. If you have symptoms and you’ve been spending
time in bars or a family member has been exposed to COVID, you should be more
cautious, even if the initial results are negative. It may be that you tested
too early and that your viral load isn’t high enough to be detected.
When Dr. Jillian Horton, an internal medicine doctor in
Winnipeg, Manitoba, started feeling ill, she was pretty sure she had COVID. Her
husband had been exposed and had symptoms, too. She decided to conduct an
experiment of one, testing herself several times over the course of a few days
to track the dynamics of the virus. “With my husband testing positive and
myself very symptomatic, I was sure I had COVID,” Horton said. “I was curious
to see what I could pinpoint in terms of when I might flip positive.”
Horton’s husband became ill on a Friday night, and that evening
she tested negative. On Saturday, she began to feel sick and tested herself
three times throughout the day. All three results were negative.
By Sunday morning, she woke up and was feeling worse. At 6 a.m.
she tested and saw a faint line on the test — what she called a “weak
positive.” She took two more tests Sunday, and both were negative.
On Monday morning, she tested again, and the test rapidly turned
positive.
What is notable about Horton’s experiment is that if she had
tested at a different time on Sunday, she may never have discovered the weak
positive. Her immune system was clearly battling the virus, as evidenced by her
two negative test results later in the day.
Horton noted that testing at the right time to catch a high
viral load was similar to putting a net in a stream. If the fish aren’t there,
you won’t catch anything. But if you time it so that the fish are plentiful,
you’ll catch your dinner.
Horton said she was concerned that too many people think the
tests aren’t working when, in fact, they are a useful tool if you understand
how to use them. They are ideal for “ruling in” COVID, but you have to consider
more information when evaluating a negative test.
“So often I hear people say, ‘The test is useless,’” Horton
said. “What my experience illustrated is that when you have symptoms, the tests
are really ‘rule-in’ tests. I think of those two days when I was so
symptomatic. I had one positive test and five negative tests. There was only
one moment in there where I was more infectious.”
Mina said that despite the limitations, people would benefit
from frequent testing any time they suspect they have been exposed, have
symptoms or want to be sure they are not infectious before spending time with a
person at high risk. He also recommends testing before you start interacting
with others again to be sure you’re not still infectious.
“These are tools that have massive benefit during a pandemic
like this,” Mina said. “They will catch you when you’re most infectious. They
will even catch you most of the time when you’re just slightly infectious. They
will catch almost everyone when they have a high enough viral load to spread.
But it won’t be perfect.”
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