In this 2020 photo, University of Washington research coordinator Rhoshni Prabhu holds up a swab after testing a passenger at a free COVID testing site in Seattle. (AP Photo/Elaine Thompson, file)

In this 2020 photo, University of Washington research coordinator Rhoshni Prabhu holds up a swab after testing a passenger at a free COVID testing site in Seattle. (AP Photo/Elaine Thompson, file)

High COVID rate causes UW medicine to limit testing

Only people with symptoms or known exposures will be given appointments.

Associated Press

SEATTLE — A Seattle-area medical system says it will soon limit COVID-19 testing appointments at its community testing sites because of an “astronomically high” positivity rate.

The Seattle Times reports UW Medicine said Thursday appointments will be limited to only those with symptoms or known exposures. Of UW Medicine’s 12 community testing sites, nine will soon start limiting appointments. Three 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, said Dr. Geoff Baird, chair of laboratory medicine and pathology at UW Medicine.

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 said the problem when the positive test rate gets very high is that all of the pools are positive, meaning they have to go back and examine the individual tests to see which was actually positive.

That significantly decreases capacity, Baird 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%.

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%.

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