US coronavirus testing is going in the wrong direction. Declines could partially explain why cases seem to be dropping.

• The US's daily coronavirus cases have declined over the last two weeks.
• The trend coincides with a decline in testing per capita in hotspots like Florida, as well as a rise in test positivity rates across 32 states.
• That means the decrease in cases could be driven by testing shortages or delays.

It seems like good news: The US's weekly average of new coronavirus cases dipped 6% this week compared to last, and 8% compared to two weeks prior. On the surface, the nation's outbreak looks to be declining after June's dramatic case surge.

But the recent downturn in cases may be "significantly overstated," according to Ian Shepherdson, chief economist at Pantheon Macroeconomics, because testing has fallen at the same pace.

Testing per capita has declined steadily in at least six states — Alabama, Florida, Iowa, Kansas, Utah, and Washington — over the last two weeks. Meanwhile, test positivity rates have risen in 32 states during the same time frame, according to data from Johns Hopkins University.

"If cases are declining with the number of tests being performed, it is important to look at the percent positivity of tests," Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security, told Business Insider. "If that statistic is increasing, that tells you that the outbreak is growing and the number of cases declining is a testing artifact."

A declining share of positive tests (or percent positivity) is a sign the US has enough testing capacity to detect the majority of coronavirus cases. But a rising positivity rate indicates that testing capacity has fallen short.

"When you see percent positivity rising, that usually means that not every case is being captured by this system," Adalja said in June. "All new cases should be something that's already on the radar of public health officials — and that's not probably the case where you have percent positivity rising."

Overall, the US's seven-day moving average for percent positivity hovers around 7.7%, down from 8.5% two weeks ago — meaning it has "has dipped only marginally," Shepherdson wrote in an analysis published Monday.

Dr. Howard Koh, a professor at the Harvard TH Chan School of Public Health, previously told Business Insider that a positivity rate between 7% and 9% is "very disturbing."

The World Health Organization recommends that governments see a 5% or lower positivity rate for at least 14 days — or lock down to get an outbreak under control. At the moment, 14 states, including Arizona, Florida, and Texas, have positivity rates above 10%. Two states — Alabama and Mississippi — have positivity rates above 20%.

"Looking at it as a nation, it doesn't really give you a good picture of this outbreak, because it's really heterogeneous and it's always been regional," Adalja said. "You've got places like Pennsylvania and New York that are bringing down the percent positivity, whereas other places are bringing it up."

The hardest-hit states are also seeing weeks-long delays in test results, and the way cases are recorded still varies from county to county. On aggregate, that makes it hard to tell if cases are indeed falling.

Testing has been backlogged for weeks

The US has ramped up testing considerably in recent months. As of Monday, the US is administering 174 tests for every 100,000 people per day, putting it second in the world behind Hong Kong in testing per capita. In total, the US has administered more than 57 million coronavirus tests.

But delays and shortages are still a problem: In its latest update on July 14, the American Clinical Laboratory Association reported that "many labs are now receiving more test orders than they are able to process in a single day."

LabCorp CEO Adam Schechter told CNBC last month that the company was struggling to raise testing capacity to keep up with the rapid spread of the virus. And Quest, another of the country's largest diagnostic testing companies, has also struggled return results quickly.

"Persistent high demand has strained our testing capacity and extended delays for test results," Quest reported on July 27. Non-hospitalized patients would have to wait a week to learn if they're infected, the company said at the time, though Quest reported on Monday that it had lowered its average turnaround time to five days.

Some states aren't listing 'probable' cases

The way states are choosing to report cases might also play a role in the falling numbers.

There are two FDA-approved methods for diagnosing an active infection: a standard PCR test, which detects the virus' genetic material, or an antigen test, which detects the spiky proteins on the virus' surface. PCR tests are generally more reliable, but the results need to be analyzed in a laboratory, so they can take several days.

Antigen tests usually produce results in less than an hour, but they have a higher chance of false negatives — and they don't always count as part of a state's official tally. Instead, a person with signs of the virus who tested positive with an antigen test is listed as a "probable" case. The Centers for Disease Control and Prevention advises all states to report probable cases, but only 27 states have done so, according to the Houston Chronicle.

The paper's analysis found that in Texas specifically, only 11 counties publish antigen test results, and those results don't factor into the state's tally. That could mean Texas' official case count is missing tens of thousands of infections.

"The only way people will be inspired to act right without government mandates is if they have the information they need to make smart choices," Texas Representative Gina Hinojosa told the Chronicle on Sunday. "And that has been just impossible to come by."

It's hard to know the scope of the outbreak without sufficient testing

President Donald Trump has attributed the US's extremely high case count to "big testing" efforts.

"We have more cases because we have tested far more than any other country," Trump tweeted on Saturday. He added: "If we tested less, there would be less cases."

But the country's high percent positivity rate suggests the virus is still spreading rampantly. So do its hospitalization numbers, which have declined recently but are still high: More than 53,000 Americans are currently in the hospital with COVID-19.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNBC that the country should ideally be "flooding the system with testing, so you really get a good handle about what is going on in the community."

A pandemic roadmap from Harvard experts suggested the US should have scaled up to 20 million tests per day by the end of July. The country has yet to hit 1 million daily tests.