In early July, Shannon Mayer started to feel a sudden tightness in her chest.
"The next day it was really hard to breathe," the 31-year-old Chicago resident told CNBC. "I got scared."
Mayer has asthma, but says she hadn't had an issue with it for years. So she decided to get a test for Covid-19. The results, she was told, would take five to 10 days, and she was instructed to quarantine while she waited. After a week, the results still hadn't been returned. And Mayer already felt better and suspected she wasn't infected, so she stopped quarantining.
"Had I stuck with it, I would have been in my house for three weeks," she said. She was tested July 1, and her results didn't come back until July 24. Luckily, she was negative. Mayer's not alone. Bethany Silva, who lives in New York City, reported a 13-day wait for her results. For Lisa Miller, in New Jersey, it was a week.
Health experts say two days or less is optimal for returning Covid-19 test results to make them useful for stopping transmission. If test results take more than three days, people are unlikely to self-quarantine and getting in touch with the people they interact with during that time — potentially spreading virus — can be difficult.
"It's really clear that if tests take more than 48 hours, you've lost the window for contact tracing," Dr. Ashish Jha, professor of global health at Harvard University, said in an interview. "I think, basically, beyond 72 hours, the test is close to useless."
A survey run by CNBC in partnership with Dynata, a global data and survey firm, suggests almost 40% of Americans had to wait more than three days for their results, rendering them — by Jha's definition — useless.
That's certainly the way Mayer felt.
"The whole purpose is to find out if I have it before it's over," Mayer said. "So that just completely defeated the purpose."
The results varied state by state. Some, like Massachusetts and South Dakota, had average turnaround times of just over two days. Others, like Arizona and West Virginia, were closer to four and a half days, on average. Indiana's average test turnaround time was more than five days.
Jha said the variation is evidence of a fragmented testing strategy in the U.S.
"It would take a national testing strategy to make sure that, if there's excess capacity in Massachusetts, but long lines in Florida, that Massachusetts could help Florida out," Jha said. "Largely we have not had a national testing strategy. The strategy out of the White House has been for every state to figure this out on their own."
Even national labs struggled to keep up with demand when cases were surging across the Sunbelt, with Quest Diagnostics saying in mid-July that its turnaround times were more than a week for non-priority patients. It has since said it's increased capacity and that results now take an average of two to three days.
Admiral Brett Giroir, the Trump administration's Covid-19 testing czar, told NBC's Andrea Mitchell this week that results that take seven to 14 days are outliers.
"In general, if you do need a test — you fall in the categories of needing a test, even for public health tracing — you're going to get that result within 48 to 36 hours," he said. Not everyone needs tests, Giroir said, and the national testing approach is "strategic testing, not shotgun testing," which he said has reversed the outbreaks in that region.
Indeed, new daily cases declined by 44% in Florida from a mid-July peak, while they're down 73% in Arizona, both on a seven-day average, according to the Covid Tracking Project, a data source run by journalists at the Atlantic.
Testing in both states is also down, by 42% in Florida and 41% in Arizona. The net result in Florida is that the positivity rate — the percentage of all tests that turn out to be positive — has remained around 18% since early July. That could mean that while reported cases have declined, the actual prevalence of the virus has not. In Texas, testing is down by half, prompting questions about whether cases are truly declining as much as the numbers would indicate, or if the decreased testing is obscuring the true picture.
"It makes examining case declines much harder to interpret," Jha said. "If cases are down by 30% but testing is down by 30%, what's happening with cases? Is it more, is it less, is it about the same? And we're all doing guesswork."
Overall, the Dynata data show that testing turnaround times have declined in the U.S. since March, from more than four days, on average, to now just less than three and a half.
That timing is still longer than Jha and others say is useful.
"The fact that six, seven months into a pandemic, we can't do a simple diagnostic test is unbelievable," he said. "The rest of the world is mostly looking at us with a state of disbelief that America can't run simple lab tests on an infectious disease that we've known about for seven months."
The survey was conducted in collaboration with Dynata, a global data and survey firm through a first-party online panel from July 30 to Aug. 10. The sample included 9,444 adults in the U.S., with approximately 200 respondents drawn from each state, plus the District of Columbia, although some of the smaller states had fewer respondents. The data were weighted to correct for known demographic discrepancies. The weighted margin of error is plus or minus 1% at the national level.