For the first time since America’s COVID-19 outbreak started, a federal public health agency released county-level testing data. On Sept. 3, the Centers for Medicare & Medicaid Services (CMS) published a dataset showing COVID-19 test positivity rates broken out by U.S. county. While this dataset has not been highly publicized, it provides key information about the state of COVID-19 testing and outbreak control across the country.

Stacker used the CMS data to show which counties in every state have the highest COVID-19 test positivity rates. These figures reflect average weekly test positivity for the week of Aug. 27 to Sept. 2. CMS has further categorized counties according to their test positivity rates: green, test positivity under 5% in the past week (or with fewer than 10 tests in the past week); yellow, test positivity between 5% and 10% in the past week; red, test positivity greater than 10% in the past week.

The forthcoming gallery further compares the positivity rates of each county to its state’s overall test positivity rate for the same week (Aug. 27 to Sept. 2). State testing figures are sourced from the COVID Tracking Project at The Atlantic. COVID-19 statistics for the most recent week of data available (Sept. 4 to Sept. 10) and cumulative case, test, and death counts, also sourced from the COVID Tracking Project, are included as well.

A test positivity rate describes the percentage of tests conducted in a particular area that returns a positive result. For example, if 1,000 people in a New York county are tested for  COVID-19 in a particular week and 10 of those people test positive, the county would have a positivity rate of 1%. Meanwhile, if 25 people in an Oklahoma county are tested and five people test positive, the test positivity rate for that county is 20%.

These positivity rates are typically reported for a short period of time, either one day or one week, and are used to reflect a region’s testing capacity over time. If a region has a higher positivity rate, that likely means either many people there have COVID-19, the region does not have enough testing available to accurately measure its outbreak, or both. If a region has a lower positivity rate, on the other hand, that likely means a large share of the population has access to testing, and the region is diagnosing a more accurate share of its infected residents.

Test positivity rates are often used as a key indicator of how well a particular region is controlling its COVID-19 outbreak. The World Health Organization (WHO) recommends a test positivity rate of 5% or lower. This figure, and a more lenient benchmark of 10%, have been adopted by school districts looking to reopen and states looking to restrict out-of-state visitors as a key threshold that must be met.

Keep reading to see which counties in your state have the highest COVID-19 test positivity rates.

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