SEATTLE (AP) — 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.