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

Testing has become one of the best weapons we have against COVID-19. There's no vaccine andno treatment, so the best way to stop the spread is quickly figure out who has it, isolate them and test their contacts.

Last week, we learnedthat a patient hospitalized inSt. John's with COVID-19hadtested negative twice, was considered recovered, and then tested positive again.

That newsnaturally has people wondering whether there's a problem with testing.

The problem may be in our understanding of what the test is telling us.

It is possible for someone to test negative andactually have COVID-19, and for someone who tests positive to not have the disease.

How testing works

The type of test being done at the laboratory in St. John's is called a PCR test.

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

When you're sick, your body sheds the virus, and one of the places it usually does that is in your nose. That's whya nurse sticks a long swab through yournose and nasal cavity, to scoop up a sample. If the sample has some 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 has to be virus in your nose for the test to find it, but that doesn't always happen, especially right after an infection.

"You don't necessarily test positive from the moment that you're infected with COVID," said Dr. Janice Fitzgerald, the chief medical officer of health for Newfoundland and Labrador.

"It takes a while for you to build up enough virus in your system to be able to test positive."

That's why a negative test after a trip or a similar result afteryou've been exposed to someone who issick isn't a path to end quarantine early.

The full 14 days is needed to see whether you'll develop symptoms. You cantest positive later,as you develop symptoms.

Testing all travellers wouldn't shorten quarantine

Some workers who are still commuting to job sites elsewhere in the province have asked whether testing upon arrival would allow them to end their isolation early.

But Fitzgerald said the only safe way to know is to wait out the full 14 days.

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

The other problem with the disease is it can move around the body. Itisn't always in your nose.

"If you test somebody 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 has estimated that up to a third of the time, someone who has the disease still tests negative.

This is why in the hospital, before someone is considered recovered, doctors want to see two negative tests, reducing the chance that the first one just missed getting part of the virus that is still there.

Sometimes a positive test doesn't mean you are infectious

When there is virus in the sample, the test is very good at picking it up.Sometimes, it's too good.

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

The body has fought off the disease and is no longer contagious, but if the swab picks up some dead virus, you will still test positive.

"You can actually have particles of RNA that are not a part of a live virus be detected and that is unfortunately something we see with these types of tests," said Fitzgerald.

Those cases meantesting positive for COVID-19 doesn't necessarilymean you're contagious.

That's what could have happened with the patientat the Health Sciences Centre. The patient had tested positive and was in the COVID unit. When his symptoms improved, two testsboth came back negative, so he was considered to be recovered.

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

One test came back positive, but then two more were negative.

Was the positive just some dead virus? Were the negatives just because the swab didn't hit the right spot to pick up the virus?

There's no way to know for sure, and so to be safe, the 99 health-care professionals who could have been exposed are self-isolating.

They've all been tested, and so far all have come back negative. All of them, though, will have to wait for the full 14 days to see if that changes.

What should we all learn?

Testing is and will be an important tool for doctors but it isn't the definitive answer that people seek in a time of uncertainty.

"A test is only as good as the clinical picture and the question you're trying to answer," said Fitzgerald.

Read more from CBC Newfoundland and Labrador