Here's Why Your COVID-19 Test Result Could Be Negative, Even If Coronavirus Is Present
When the coronavirus enters the body, usually through the nose, mouth or eyes, it takes over cells and starts to multiply. By analyzing a molecular or polymerase chain reaction (PCR) test, a lab technician may find out how much of the virus you have.
Back in July, I experienced one of these PCR tests. I sat down on a chair, and a nurse pulled out a clean, long stick with a brush on one end. I knew it needed to go deep into the nasal passage.
"OK so deep breath in, mouth closed, exhale mouth open," the nurse instructed after telling me to put my head back, and then twirled the stick around to make sure she got enough specimen. After one nostril was done, I found out we weren't finished yet.
"I’m going to go into your other nostril," she said.
After I took a short breath, she twirled for some 15 seconds. Luckily the whole process didn't take long, but that wasn't the end of the road for my test.
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The specimen then went to a lab in South Florida. If a sample has viral genetic material in it, then it goes through a process of amplification cycles to see if the coronavirus is detected, and that number of cycles is called the cycle threshold or CT value.
My specimen turned out not to have virus in it. But if a specimen has a lot of virus in it, it's faster to spot the genetic material, so the cycle threshold will be lower.
"The cycle thresholds will be lower because you can see it sooner during this polymerase chain reaction," explained Michael Teng, professor of medicine at the University of South Florida. "We assume that when you have, you know, these really low cycle thresholds that you are making a lot of virus."
If you’re making a lot of the virus, Teng said it is typically detected by the 20th cycle. If it’s not detected by the 37th or 38th cycle, he added, the lab will likely call it inconclusive or negative.
The amount of virus you have is known as the viral load, "which is a direct correlation of being exposed to the virus and having the virus," said Bindu Mayi, a professor of microbiology at Nova Southeastern University.
"The virus having had enough time to multiply, to replicate, make more copies of itself — that’s all the virus wants to do. It just wants to make more copies of itself, and that takes a little bit of time."
At the time of testing, a negative test means you didn’t have the virus or you didn’t have enough of the virus for it to be detected.
To play it safe, Mayi recommends we assume that people around us in public may be carrying the virus. She said she limits her trips to the supermarket to once a week or once every ten days, and goes just as the store opens.
But many folks work in retail stores and can't avoid extended inside exposure.
"So our best bet is prevention across the board," Mayi said. "This is why the universal masking becomes so important. This is why the hand hygiene is so important. This is why the physical distancing is so important."
The sooner we all take these preventative steps, the sooner we’ll stop transmitting the virus, she added.
Once students start in-person learning again in South Florida, the risk of spread will increase, said Aileen Marty, a professor of infectious diseases at Florida International University.
"But how much it increases depends on the viral load in our community, and so if we’re doing everything possible to keep that viral load down," she said, then, outbreaks on campus are likely to be smaller and less frequent.
The school district in Monroe County has reopened for in-person learning, Palm Beach County plans to reopen Sept. 21 and Broward could reopen Oct. 5 — Miami-Dade County has not announced when buildings could reopen for instruction.
Meanwhile, we still don’t know exactly how big a dose of the virus it takes to get another person infected. USF's Professor Michael Teng says we can also help stop transmitting the virus by, for instance, following the arrows on the floor at places like Publix and Walgreens, which direct shoppers which way to walk down the aisles.
"Because if you have two people coming towards each other, the airflow kind of gets mixed up in the air, doesn't move anywhere," Teng said. "But if everybody is going the same direction, the air will actually flow through the aisle that way."
Teng, a virology expert, also recommended opening windows to improve ventilation and using ceiling fans to circulate the air, "making sure that you're dispersing any clouds of virus coming out of people when they breathe," he said.
"We don’t have any one thing that will take care of the virus, so we need to do everything."
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