One week ago, 300 healthcare workers at the Hospital of the University of Pennsylvania gave blood as part of a new test to find out if they have, or ever had, been infected by the novel coronavirus.

It is called antibody testing, and it will expand to 1,000 workers in the Penn Medicine health system over the next couple weeks to determine how much the virus has spread among hospital workers.

The test is different from one conducted on ill patients because it can determine if a person has ever been infected by the virus.

The initial round of blood samples in the Penn study proved heartening: Not many of the 300 workers tested positive for antibodies that indicate infection from the novel coronavirus, according to the lead researcher for the test, Scott Hensley.

"It's an encouraging sign that there is not widespread virus in our hospitals," said Hensley, an associate professor of microbiology at Penn, though he added that it is too early to interpret the data in any predictive way.

The Penn antibody test was developed in-house and provides very specific results. Hensley credited a donation weeks ago from Philadelphia 76ers player Joel Embiid and the team's owners with jumpstarting his lab's testing.

That's not the case for the dozens of other antibody tests starting to be used across the country on a broader population of patients. Many of those tests were approved without stringent clinical trials normally performed by the Food and Drug Administration.

Antibody testing looks for signs that the coronavirus was inside a person's body. Determining how many people have been infected by the virus, beyond those who became ill, could greatly expand our ability to slow its spread and our understanding of how the virus works, infectious disease experts say.

Still, they caution against expecting immediate success on a large scale.

"There are so many tests out there right now. It's sort of like the Wild West," Hensley said.

Dozens of new antibody tests, called "point-of-care rapid tests," provide patients with a plus or minus, meaning positive or negative for antibodies that would reflect whether the coronavirus has or had been present in blood.

Many could prove to be unreliable, according to Dr. Thomas Fekete, an infectious disease expert and chair of medicine at Temple University's Lewis Katz School of Medicine.

He said trustworthy testing on a broader scale could take a month or so to find.

"We want to make sure the test is picking out the exact virus," Fekete said. "It may be a little difficult. It make take a little while to shake out."

Finding a reliable test that can be taken by hundreds of thousands of Americans, and with quick turnaround in results, could prove key to figuring out the true spread of the infection and how to stop it, Fekete said.

Many people are believed to have been infected, but suffered little or no effects.

Still, it remains unclear why the virus has such wide-ranging effects, and a reliable antibody test won't end the lockdown by itself.

There remains much mystery to the virus, including whether someone who was previously infected can be infected again and why some people suffer more severe reactions than others.