ST. PAUL — Marketed as an easy-to-use and convenient way to self-diagnose COVID-19, over-the-counter rapid tests seem hard to come by.

Websites for Walgreens, CVS and other pharmacies list brands of the tests as out of stock. Even in Minnesota, where residents have easier access to free tests than neighboring states, rapid tests appear to be in short supply.

"Having been to a couple pharmacies recently, when we were looking for some of those rapid tests, they are sold out at a lot of pharmacies in Minnesota," said Dr. Amy Karger, an associate professor of laboratory medicine and pathology at the University of Minnesota Medical School.

Amid a surge in COVID-19 infections driven by the coronavirus delta variant, a renewed demand for the tests has made them hard to find. The makers of one rapid test brand in August told The New York Times that demand was 1,000 times what it had forecast.

The Minnesota Department of Health last week cited the demand for tests in announcing the launch of two "semi-permanent" testing sites in Lino Lakes and Minneapolis. The move stands in contrast to the closure of several state-run testing sites earlier in the summer amid a decline in new cases.

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Testing rates in Minnesota, which help to gauge the spread of COVID-19, seem to still be strong. Health department spokesperson Doug Schultz said in an email that labs handling tests last month processed between 10,000 and 30,000 samples a day.

"We don't really know how many (over-the-counter) tests are being used daily in MN, but it is not likely to be significant in comparison," Schultz wrote.

Schultz said the health department is also "continuing to evaluate to determine if the overall positivity rate is being affected by the absence of reports of at-home test results."

With the exception of the free saliva tests Minnesota offers, at-home tests aren't generally reported to the health department. Even a case reported by a doctor to the Minnesota health department based on a patient's results from an over-the-counter test performed at home, according to Schultz, would not be recorded.

"There are a number of reasons why," Schultz said, some of which have to do with inconsistencies in information collected by test makers, such as ZIP codes and birth dates. "And we really don’t have any way to validate the test result."

A patient calling their doctor to report their positive result, on the other hand, would likely be asked to come in for a follow-up exam, Schultz said, although protocols may differ from provider to provider.

Karger, who helped advise state officials in their efforts to test for COVID-19, had a similar outlook.

"Generally speaking, the data that department health is collecting is likely accurately going to reflect what's going on," she said. "Even if they're not capturing all the tests necessarily in the state, they can look at those rates of positivity to kind of get a sense of whether we're going in the right direction or not."

Test makers and health officials still recommend consulting a doctor and alerting close contacts if you do test positive on an over-the-counter product.