Here’s why a positive test doesn’t always mean you have COVID-19


Newfoundland and Labrador has tested more than 9,500 people, but a negative or positive result does not always provide a definitive answer. (Graham Hughes / The Canadian Press)

The tests have become one of the best weapons we have against COVID-19. There is no vaccine or treatment, so the best way to stop the spread is to quickly determine who is infected, isolate them, and test their contacts.

Last week, we learned that a patient hospitalized in St. John’s with COVID-19 had tested negative twice, was considered cured, and then tested positive again.

This news naturally leads people to wonder if there is a problem with the tests.

The problem may lie in our understanding of what the test tells us.

It is possible for a person who tests negative and in fact to have COVID-19, and for a person who tests positive to not have the disease.

How tests work

The type of test performed at the St. John’s laboratory is called a PCR test.

It detects RNA, or ribonucleic acid, which is the genetic material that makes up the coronavirus.

When you are sick, your body clears the virus and one of the places it is usually found is in your nose. This is why a nurse passes a long swab through the nose and nasal cavity to collect a sample. If the sample contains part of the virus, the test detects it.

Unfortunately, it doesn’t always work that way.

Sometimes you have it, even if the test says negative

There must be a virus in your nose for the test to find it, but it doesn’t always happen, especially right after an infection.

“You are not necessarily positive from the time you are infected with COVID,” said Dr. Janice Fitzgerald, Chief Medical Officer of Health for Newfoundland and Labrador.

Dr. Janice Fitzgerald said there are times when someone can be positive for COVID-19, even if they are no longer contagious. (Government of Newfoundland and Labrador)

“It takes a while for you to accumulate enough viruses in your system to be able to test positive. “

This is why a negative test after a trip – or a similar result after being exposed to a sick person – is not a way to end quarantine early.

The full 14 days are necessary to see if you will develop symptoms. You can test positive later because you develop symptoms.

Testing all travelers would not shorten quarantine

Some workers who still commute to workplaces elsewhere in the province asked if screening on arrival would allow them to end their isolation sooner.

But Dr. Fitzgerald said the only sure way to find out is to wait the full 14 days.

“You can be falsely reassured by a negative test,” she said.

The other problem with the disease is that it can move around the body. It’s not always in your nose.

The PCR test is extremely sensitive to detecting the virus in a sample, but the swab does not always find the virus in someone who can be effectively infected. (Steve Helber / Associated Press)

“If you test a person with a nasal swab at a time when they have symptoms in their lungs, you may not get a positive result,” said Fitzgerald.

British Columbia estimates that up to a third of the time someone with the disease still has negative results.

This is why in the hospital, before someone is considered cured, doctors want to see two negative tests, which reduces the risk that the first one has missed part of the virus that is still there.

Sometimes a positive test doesn’t mean you’re contagious

When there is a virus in the sample, the test is very effective in detecting it. Sometimes it’s as well good.

Even after a person has recovered, they can still be positive.

The body has fought the disease and is no longer contagious, but if the swab detects a dead virus, you will still be positive.

The government of Newfoundland and Labrador says it has enough swabs and reagents to test more people. (Maggie MacPherson / CBC)

“You can actually detect RNA particles that are not part of a living virus and that is unfortunately something that we see with these types of tests,” said Fitzgerald.

These cases mean that a positive test for COVID-19 does not necessarily mean that you are contagious.

This is what could have happened with the patient at the Health Sciences Center. The patient had tested positive and was in the COVID unit. When his symptoms improved, two tests both returned negative, so he was considered cured.

The patient was moved out of the COVID unit, and when it looked like get was getting sicker, he was tested again.

One test came back positive, but two others were negative.

Was the positive just a dead virus? Were the negatives simply due to the fact that the swab did not reach the right place to detect the virus?

There is no way to know for sure, and therefore to be safe, the 99 health professionals who may have been exposed isolate themselves.

They have all been tested and so far all have returned negative. All, however, will have to wait 14 full days to see if this changes.

What should we all learn?

Tests are and will be an important tool for doctors, but it is not the definitive answer that people seek in times of uncertainty.

“A test is as good as the clinical picture and the question you are trying to answer,” said Fitzgerald.

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