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An embattled COVID-19 test that senses the presence of the virus by detecting antibodies to it has been found to be accurate enough that its Minnesota distributor can sell it, just as other tests are being removed from the market.

Premier Biotech, a Minneapolis drug-testing firm that has pivoted to distributing a Chinese-made test for detecting COVID-19 antibodies, plans to ramp up sales in coming weeks. The Food and Drug Administration granted an emergency-use authorization (EUA) for the test on June 4, and on Thursday posted the results of an independent performance assessment by the National Cancer Institute.

“For me, having the results published was a big deal, so the rest of the country could know what we knew from the beginning, which is that this is a good test,” Premier Biotech CEO Todd Bailey said Sunday.

Other tests are being taken off the market. One, made by BioMedomics and sold by major medical technology firm BD, was removed in early May after the FDA updated the rules on performance characteristics needed to get EUA.

A second-generation test is in the works. Tests made by Phamatech and Tianjin Beroni Biotechnology Co. were also voluntarily withdrawn.

Researchers and medical labs generally use two overarching kinds of tests for COVID-19 — serology tests and molecular tests.

Serology tests like the one distributed by Premier Biotech react to the presence of specific antibodies that a person’s immune system makes to fight off the novel coronavirus that causes COVID-19.

Antibody tests on the market today including Premier’s cannot be used to diagnose a specific case of COVID-19. But they can be used in “seroprevalence” studies on overall rates in well-defined populations.

Scientists are still trying to refine their knowledge about how much time needs to pass before people have detectable numbers of antibodies.

One early study of antibody testing, not yet peer-reviewed, looked at 173 patients with COVID-19 in China and found the rates of correct detection of the IgG and IgM antibodies was at its highest at 16 days after symptom onset.

Last week, Mayo Clinic announced availability for a new antibody test produced by Rochester-based Vyriad to look for the presence of neutralizing antibodies.

Molecular tests, on the other hand, are the gold-standard method of diagnosing an active COVID-19 infection. Often called PCR tests, these tests detect the presence of viral genetic material in swabs taken deep from the throat or nose.

Their accuracy peaks around eight days after exposure, a study-of-studies in the Annals of Internal Medicine shows.

The Minnesota Department of Health says diagnostic testing has been used to confirm 30,471 cases of COVID-19 in residents since early March, including 311 cases added to the tally Sunday.

Of those, about 26,100 people have passed a 14-day self-monitoring period and no longer need to self-isolate.

In Minnesota, 1,298 people have died from COVID-19, including 1,030 residents of long-term care and assisted living.

The vast majority of fatalities happened in people 50 or older. Underlying health conditions increase the risks, including chronic lung disease, uncontrolled severe asthma, serious heart conditions, immunocompromise, obesity with BMI greater than 40, diabetes, chronic kidney disease requiring dialysis, and liver disease.

Both the rate of new deaths and the rate of newly confirmed cases has been slowing in Minnesota for two weeks despite increased PCR testing and large-scale civil rights demonstrations over the death of George Floyd.

Nationally, the United States has recorded more than 115,000 deaths among more than 2 million confirmed cases.

The number of new cases is rising in states including California, Arizona, Texas, Florida and Georgia, with some of the rise due to increased testing. Case counts are declining in New York, New Jersey, Illinois and Massachusetts, according to the New York Times’ closely watched case tracker.

Early in the pandemic, the FDA responded to the sudden and overwhelming demand for COVID-19 tests in the U.S. by lowering the bar to entry for many.

PCR tests started to be sold with only minimal prior validation by their manufacturers, while more than 100 antibody tests flooded the market with no direct oversight by the FDA.

As a result, many tests suddenly became available with scant evidence supporting their quality.

The Associated Press reported Sunday that even now, months into the pandemic, “no one really knows how well many of the screening tests work.”

The U.S. House Subcommittee on Economic and Consumer Policy entered the fray in April, demanding to know why the FDA allowed unproven tests to go to market and why some tests’ claims of accuracy on packaging were not reproduced in independent testing. The subcommittee claimed credit for forcing the FDA to require antibody test makers to submit data and get emergency use authorization.

One of the tests singled out for extra scrutiny was the one sold by Premier Biotech.

A letter from the subcommittee to the company CEO alleged that academic researchers in California had found far lower rates of accuracy for the test in an unpublished study than what was advertised on the product label.

The subcommittee’s letter said the test correctly identified the presence of a specific antibody called IgG only 53% of the time in 129 validation tests. An April 28 letter from the subcommittee said the test’s sensitivity was “deeply troubling.”

But last Thursday the FDA publicly posted the results of federally funded performance assessments done at the Frederick National Laboratory in Maryland that found the Premier Biotech test correctly identified the IgG antibody 93% of the time in 30 samples. A second antibody, IgM, was positively identified in all 30 tests. The test didn’t return positive results for any of the 160 negative samples used on it.

Virtually all of the tests on the FDA’s web page for validated antibody tests today have positive-detection values between 90 and 100%.

Bailey said Sunday that the independent review, which used specimen-handling methods described on the product label, agreed with the validation results Premier had seen from other organizations.

“We were extremely excited, but not surprised,” Bailey said. “We knew the test was performing well. But having the [federal laboratory] do the study themselves as a federal study, that is the best part.”

Like many antibody tests, the Premier Biotech test is only validated today for use in certified labs for high- and medium-complexity work. The company is working on research to show it can also be accurate at the point-of-care using blood drawn from a finger stick, but that work is ongoing.