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Why your COVID test comes back positive or negative

When your nasal swab gets analyzed, the result is a scientific value called the cycle threshold.

DENVER — Just like the U.S. Postal Service has deadlines to get your Christmas gift mailed out on time, we are approaching the day to get your COVID test done in time to know your status for your family's holiday dinner.

Just remember that a COVID test only tells you about your status on the day you took the test.

When your nasal swab gets analyzed, it comes back with a scientific value that determines if you are deemed positive or negative.

That value is called the cycle threshold.

"'Cycle threshold' is basically a term that we use when we're doing a molecular test. When you run these molecular tests for these viruses, it gives you a number. It doesn't give you a plus or a minus," said Dr. David Beckham, UCHealth virologist and associate professor of infectious disease at the University of Colorado Anschutz Medical Campus.

Just like a pregnancy test, the positive or negative result is really a value that is converted into an easy-to-understand symbol.

Why are we using the term "cycle threshold?"

The NFL just changed its protocols for when a vaccinated player who tests positive for COVID can come back and rejoin the team.

One of the new ways for a player to be cleared to return is to have two negative PCR tests or have their PCR tests have a cycle threshold value of 35 or greater.

Basically, the cycle threshold value is the number of times your sample is cycled to detect the virus. A higher number means either you do not have the virus or you have so little active virus that it is unlikely you are infectious, thus your sample is deemed negative.

A number lower than 35 means you have enough active virus that you are likely infectious, and your test is considered positive.

"You kind of have to pick a number where you say, 'OK, above this number it's just background noise, and below this number, based on our controls and other tests that we've done, below this number is a real positive and it's really there,'" Beckham said.

For example, Mako Medical is one of the free test providers in Colorado. According to a company spokeswoman, the lab uses a cycle threshold of 35. That means when Mako Medical runs your sample, anything greater than 35 comes back negative and anything lower than 35 comes back positive.

Summit BioLabs runs the tests for many of the COVID Check Colorado free testing sites. According to the company, its cycle threshold is 36.

"Every diagnostic test that we do has a threshold number associated with it -- a value associated with it -- and we just determine what's positive and negative, so this is just the details of how diagnostic science works," Beckham said.

The simple answer for why different labs have different cycle thresholds is because they have different machines.

Because the number, like 35 or 36, can differ from lab to lab, it could mean some people who have enough virus to be detected but may not be infectious anymore, may produce positive results.

"The lower you go with that number, the more likely you are to catch what we call true positives," Beckham said. "You want to catch as many potential positives as possible, so sometimes you have to -- when you're setting these numbers -- be OK with calling some people positive, even if they're negative."

Beckham said that he uses the cycle threshold value to determine how a patient in the hospital is progressing.

"When we're in the hospital and I'm seeing patients that have COVID and have been there for a month or two months, I'll often call the laboratory and get the CT value so I understand, 'OK, how much target is probably there?'" Beckham said. "We actually have a rule in a lot of our hospitals, that if you're recovering and you're within 60 to 90 days, we're not going to retest you because you're going to come back positive and it doesn't necessarily mean you're infectious."

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