The University of Minnesota and Mayo Clinic are now offering antibody tests to find out who has recovered from COVID-19 — testing that Gov. Tim Walz views as critical to the state’s recovery from the ongoing pandemic.

Use of the U’s new test is expected to start this week at M Health Fairview’s Bethesda Hospital in St. Paul — with priority going to doctors, nurses and others in the facility that exclusively treats COVID-19 patients. Mayo in the past week has used antibody testing on 86 workers or patients.

Walz on Monday said he is challenging these institutions and others to dramatically ramp up in-state production of serological antibody testing, which could identify people who are no longer at risk of spreading or contracting the virus.

“We have to have an accurate way to understand who is infected currently and who has been infected,” Walz said.

COVID-19 has been lab-confirmed in 1,650 cases and has caused 70 deaths in Minnesota. Without sufficient testing, the state’s primary defense has been social distancing to reduce face-to-face contact and exposure risks — starting one month ago with Walz’s closure of dine-in restaurants, entertainment destinations and schools, and then the existing stay-at-home order.

Walz on Monday extended a peacetime emergency declaration until May 13 — giving him another month of administrative powers to take such steps. He also announced a state executive hiring freeze and 10% pay cuts for himself and his commissioners — the latest sign of the economic hardships amid the pandemic that include nearly 427,000 people applying for unemployment benefits.

Republican lawmakers continued to challenge the prolonged stay-at-home order, arguing that recent state data modeling projected an equivalent death toll and hospital surge if the restrictions were applied only to the elderly and high-risk groups.

“If we can begin the process of repairing our economy while still protecting those most vulnerable, we should have that conversation. Let’s let the numbers guide our work,” House Minority Leader Kurt Daudt, R-Crown, said.

The governor said his extension of the statewide stay-at-home order until May 4 was partly to buy time for hospitals to prepare, but also for the expansion of antibody and diagnostic testing in the next few weeks. Together with aggressive contact tracing and isolation of people who have been exposed to the virus, testing would allow the state to scale back and maintain restrictions on only those high-risk groups, he said.

“The problem is we are not ready … yet because we don’t have the testing in place,” he said.

Walz said the state needs to get to a point of 40,000 diagnostic tests per week, even though the state’s public and private labs have conducted only 38,000 of the tests of nasal or throat swabs so far. The serological tests are different in that they look after the fact for antibodies, the proteins produced by the human immune system to neutralize viruses.

Scientists don’t yet know if people can be reinfected by the coronavirus, but they believe based on studies from the 2002-03 SARS outbreak that people might have immunity for months or even years.

Whether the U and Mayo are in a position to produce enough antibody tests to meet the governor’s goal is unclear. The U’s lab should be able to make at least 100,000 tests. Mayo already has the capability to conduct 10,000 antibody tests per day and may soon double that capacity.

Public demand is expected to exceed capacity, in part because 80% of COVID-19 infections cause only mild symptoms — leaving many people in limbo wondering whether they were infected.

The U plans to make its antibody tests available for statewide surveillance and other uses, but for starters it is using them for health care workers who face daily exposure risks and could benefit from information about their COVID-19 status, said Dr. Jakub Tolar, the U’s vice president for clinical affairs and medical school dean.

“I need to protect my health care workers,” he said. “I need to figure out how to get this to them on the front line.”

Among Mayo’s 86 tests so far, 12 found antibodies in patients, suggesting they had recovered from COVID-19, said Elitza Theel, director of Mayo’s Clinical Microbiology Laboratory.

The U.S. Food and Drug Administration on April 1 issued emergency use approval for one type of antibody test by Cellex Inc., but Walz said he has concerns about the national demand for that test and the ability of the federal government to deliver a reliable quantity to Minnesota.

The federal government delivered 15 of Abbott Labs’ rapid diagnostic testing machines last week but only enough chemicals to perform 120 tests with them, he noted.

While most COVID-19 cases are mild, as many as 5% of infections cause severe respiratory symptoms that require intensive hospital care or ventilators to help patients breathe. Minnesota is projected to run short of ICU beds, but Walz said the stay-at-home order buys hospitals time to add more beds as well as tests.

The good news is that the U-derived antibody test really appears to work — as long as blood serum is drawn from patients well after their infections, said Marc Jenkins, director of the U’s Center for Immunology.

The test correctly produced negative results when applied to 200 samples of serum — the clear liquid left after red and white blood cells and clotting factors are removed — that were collected before the COVID-19 pandemic, he said. When applied to serum from 50 lab-confirmed COVID-19 cases, the test found antibodies in all of the patients who had been infected at least 15 days.

The test was less accurate when looking at people within 10 days of infection, because it takes time for them to produce antibodies in measurable quantities, Jenkins said. But in each patient, the antibodies eventually emerged.

“We actually could observe them start to make antibodies,” Jenkins said. “That’s really remarkable evidence that the test is measuring what we want it to measure.”

Testing also is fairly fast — with results coming back in as little as four hours.

Theel said her lab only advises testing after 10 days of symptom onset or exposure to a COVID-19 case.

While the test has proved remarkably accurate, it isn’t foolproof, she said. “Importantly, some individuals, particularly those who are immunosuppressed, may never develop a detectable immune response.”