Biochemistry senior Jessica Van Lengerich sat eating a cookie at McNamara Alumni Center on Tuesday after donating blood for a Red Cross blood drive.

This was her fifth time making a blood donation and she said she views it as an important responsibility.

“Other people need it and I have it,” she said. “If I were to ever get into some car crash or something, I would appreciate whoever donated.”

Another donor, hospital pharmacy resident Lindsey Kelley, said others might need her blood even more than she does.

“Why wouldn’t you donate? It is so easy,” she said.

It might not be as easy as people previously thought, according to a study published in July and conducted by Dr. Jeffrey McCullough, a professor in the medical school’s department of laboratory medicine and pathology.

McCullough used a new method of calculation to determine that only 37 percent of Americans are eligible to donate blood.

This number is much lower than the 59 percent previously calculated, which only used age as a determining factor for eligibility.

“No one ever considers other reasons why blood banks defer donors,” McCullough said.

The study was conducted over six months and started with researchers checking public health databases of 18- to 65-year-old Americans for 32 previously recognized factors that disqualify a person from donating blood.

McCullough said the research team had to differentiate between permanent and temporary deferral characteristics. They finally settled on the 37 percent figure.

“Blood banks knew adding additional tests and deferral criteria were shrinking the donor pool, but it was kind of astonishing to us that no one had ever added it up,” McCullough said. “What we were doing was not exactly rocket science, but no one had ever done it.”

Laura Kaplan, manager of marketing and communication for Memorial Blood Centers, said the blood bank industry is conscious of the fact that not everyone can donate blood.

“It is really difficult to determine eligibility,” she said. “There are so many variables.”

Deferral factors include such things as HIV/AIDS, past trips to certain areas of the world and even something as simple as the common cold.

Kaplan also said there are industry safeguards to protect the blood supply.

Since no one is paid to donate blood, she said, there is less incentive to lie about medical history. She said all blood is also tested for safety reasons.

Shortly after donating blood Tuesday, graduate student Andrea Nelson said she agreed with the restrictions on blood donation.

“It’s best to make sure people have clean, healthy blood,” she said.

Kaplan said despite the restrictions, people are still donating.

“We generally say that about 5 or 6 percent of the eligible population donates,” she said. “These few people are saving a lot of lives.”

McCullough, however, said he disagrees with that figure.

“That number has been floating around for the last 25 years, but it is not based on any data,” he said. “That is what finally got us to do the study. It is something that has been talked about and everyone believes it, but no one has ever really tried to demonstrate it.”

To do that, McCullough said he hopes organizations such as the National Heart, Lung and Blood Institute can help fund further research.

“The number we have is the number of the U.S. population that is free of the exclusionary factors,” he said, “but of those numbers, we don’t know how many of them actually fit the profile of an existing donor.”

By comparing actual donor demographics to the 37 percent of eligible donors, McCullough said he hopes to nail down a more precise number of Americans who actually donate.

“I think the 5 percent number is quite an underestimate,” McCullough said.