Sept. 10 (UPI) -- A widely available, inexpensive and easy-to-use test accurately identifies COVID-19 antibody levels in potential convalescent plasma donors, according to a study published Thursday by the Journal of Clinical Investigation.
The ELISA -- or enzyme-linked immunosorbent assay -- test has been used for decades and generates results faster than the virus neutralization titer tests currently being used, which are costly and only canbe processed by some labs, researchers said.
Using blood samples from more than 2,800 donors, the ELISA approach produced comparable results to virus neutralization titer tests at least 80% of the time.
The test successfully identified donors with COVID-19 antibody levels above a "magic number" of 1,350, which is thought by researchers to be the minimum amount needed to offer protection against the virus.
"Will there be modifications to this finding? Absolutely. That's what science is -- but I think we're one step closer to understanding the level of antibodies needed to protect against this virus," study co-author Dr. James M. Musser told UPI.
"I'll put it this way: If I were being vaccinated today, I would want my titer to be above that 1,350 number," said Musser, a pathologist at Houston Methodist Hospital.
Antibodies are proteins made by the human immune system to fight off infection. Because they are found in the blood, convalescent plasma -- or the liquid component of blood -- has long been used to treat infections, Musser said.
According to theory, plasma donated by people who have successfully fought off a virus can be collected and transfused into others to bolster their immune systems and, hopefully, protect them from serious illness.
In the case of COVID-19, the U.S. Food and Drug Administration last month issued an emergency use authorization for convalescent plasma donated by people who have recovered from the virus as a treatment in hospitalized patients -- provided that donors have sufficient antibody levels.
However, because of limited access to virus neutralization titer tests, most donor plasma virus antibody levels remain unknown before transfusion.
For their analysis, Musser and his colleagues at Penn State, the University of Texas at Austin and the U.S. Army Medical Research Institute of Infectious Diseases developed an ELISA-based test that measures two known COVID-19 antibodies: anti-spike ectodomain and anti-receptor binding domain IgG.
They evaluated the ELISA test's accuracy at measuring specific antibodies against COVID-19 using blood samples more than 2,800 employees at Houston Methodist Hospital.
Both the ELISA and virus neutralization titer tests revealed convalescent donors maintain high levels of immunity for several weeks, even after multiple donations, the researchers said.
In addition, donors who experienced shortness of breath while infected with COVID-19 and those who were hospitalized or had severe disease had higher levels of neutralizing antibodies, the tests showed.
They also identified 27 study participants with mild disease who had high enough antibody titers to donate usable plasma.
The FDA used the findings of this study as part of its evaluation for the emergency use authorization for convalescent plasma, Musser said.
"We think our findings give us a baseline or floor for the amount of antibodies needed to offer protection against the virus," he said.
"More research still needs to be done, but this number will likely be an important part of identifying potential plasma donors and determining the effectiveness of any new vaccine."