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Too many Americans are testing positive for COVID-19 as a percentage of all those screened for the disease—a worrisome indicator that many states still aren’t testing enough of their infected residents.

Arizona, Florida, South Carolina, and 25 other states all are seeing spikes in their “positivity rates,” the percentage of people who test positive for the coronavirus out of all those tested, according to Johns Hopkins University’s COVID-19 tracking project.

Positivity rates should be under 5% for at least 14 days in areas with adequate testing, according to the World Health Organization. But Arizona now has an eye-popping positivity rate of 25.3%, meaning that one in every four people tested has COVID-19.

Higher positivity rates “may indicate that the state is only testing the sickest patients who seek medical attention, and is not casting a wide enough net to know how much of the virus is spreading within its communities,” according to Johns Hopkins. Lower rates indicate that people with mild and asymptomatic cases are also being screened for the coronavirus and that the state “is testing enough of its population to make informed decisions about reopening.”

Florida’s positivity rate is 18.7%, and South Carolina’s is 16.6%; other states that have recently seen surges in confirmed daily coronavirus cases, including Texas and Mississippi, also have positivity rates above 13%.

Connecticut, Maine, and the onetime coronavirus hotspot of New York have the lowest positivity rates, each below or just above 1%, according to Johns Hopkins.

Also worrisome, the overall U.S. positivity rate is mounting again. It fell to as low as 4.2% in June, according to the COVID Tracking Project. But it had risen to 7.3% as of July 6, even as the overall number of tests given increased.