UW Medicine will soon start limiting its COVID-19 testing appointments to include only those with symptoms or known exposures because of an "astronomically high" positivity rate that's slowing the testing process, the health care system announced Thursday.
Of UW Medicine's 12 community testing sites, nine will soon start limiting appointments. Three, however, including in Ballard, and at Seattle City Hall and Lake Sammamish State Park, will close temporarily.
The high positivity rate — measuring more than 40% at some South King County testing sites — is creating a challenge in UW Medicine's testing process and slowing scientists' ability to parse out which samples are actually positive, Dr. Geoff Baird, chair of laboratory medicine and pathology at UW Medicine, said in a news briefing Thursday morning.
UW scientists normally opt for a "pool testing" system — common throughout the country — to speed up the testing process, which means they take four or five samples from testing sites, extract a small portion of each, then mix them together in one vial for testing.
"If that sample was negative, all of those samples would be considered negative because the test is extremely sensitive," Baird said. "So we would really have done the work of four or five tests with just one test. It was a way to increase capacity."
He continued, "The problem when the positive test rate gets very high, though, is that all of the pools are positive. If one or two of them are ... positive, we'd still have to go back and test all four individually to see which (individual sample) was positive — so we can't do pooling anymore."
That significantly decreases capacity, he said.
At the county's Auburn collection site, the positivity rate was 49% earlier this week, he said. In the past, scientists have seen 5% to 10% positivity rates at Seattle sites in between surges, and at previous peaks positive rates have been around 20% to 25%, Baird said.
As a result, UW Medicine will only accept appointments for people who have symptoms of a respiratory illness or those who have a known exposure to someone who's tested positive for COVID-19. People without symptoms who want to get tested before travel or gatherings, for example, will temporarily no longer be able to make appointments at UW testing sites, Baird said.
The change will go into effect next Tuesday, and last until scientists have confirmed samples' positivity rate has fallen back down to an average of below 10% to 15%, Baird said.
Overall, most recent data shows the positivity percentage is about 25% — while test sites in South King County are between 40% and 50%, others in the Seattle area are much lower — so the rate will need to go down by about half for labs to "operate at full strength" again, Baird said.
Anyone who has an appointment before next Tuesday will still be able to hold onto their timeslot regardless of symptoms or exposure.
"This could change in a matter of days," he said.
Baird said the lab has generally been receiving 10,000 to 15,000 samples per day for most of December, with a pre-Christmas peak at 20,000 tests per day. If scientists want to get results out within 24 hours or less, they'll have to scale back capacity to about 8,000 to 10,000 tests per day, Baird said.
"We could accept more tests, but we wouldn't get to them in a meaningful time," he said. "... In the last few days, we've gotten to the point where our turnaround times sometimes exceeds two days, and medically speaking, a COVID test that's not back for several days just isn't terribly meaningful because someone could go on and spread the virus and they wouldn't have the important information they need to make the decision about whether or not to quarantine or limit contact, isolate, etc.
"We are making this decision because we want to keep the medical value of the testing intact," he added.
Baird reminded the public Thursday that "testing is one tool in the toolbelt" for fighting viral spread, but that vaccines, distancing and limiting social gatherings are perhaps more important.
"COVID is not transmitted because of a lack of testing," he said. "COVID is transmitted by human behavior with people interacting and being in close contact."
Still, he acknowledged that the health care system will not be able to as accurately keep track of the total number of infections as long as appointments are limited.
But, he said, the city and state have long known infection numbers are being undercounted because not everyone who gets sick gets a test. In addition, those who test positive with an at-home rapid test don't always report their results to public health agencies.
He added that the positivity percentage is still a "very good surrogate for what's out there."
