Whether the gender of a blood donor could affect the recipient's survival was an unanswered question in medicine. Until now.

"Some observational studies had suggested female donor blood might be linked with a higher risk of death among recipients compared to male donor blood, but our clinical trial found that isn't the case," co-lead author Dr. Dean Fergusson, a senior scientist at the Ottawa Hospital in Ontario, Canada, said in a hospital news release.

The U.S. National Heart, Lung, and Blood Institute identified this question as a research priority in 2015.

To study this, researchers began a large, innovative clinical trial with more than 8,700 patients.

"To answer this question definitively we needed a large, randomized clinical trial, but those studies are incredibly expensive," said Dr. Michaël Chassé, co-lead author of the study.

"By embedding this trial in real-world practice and using practical methods, we answered this question for a fraction of what a trial would normally cost," added Chassé, an associate professor at the University of Montreal.

Instead of using typical trial methods, the researchers enrolled every adult patient at the Ottawa Hospital who might need a transfusion, and then randomly assigned them to receive male or female blood. The study team then collected data from existing hospital databases and provincial registries.

The research team estimated that using typical trial methods would have cost $9 million, compared to the $300,000 their approach cost.

The study excluded patients who did not have an Ontario Health Insurance Plan number and those who were massively bleeding and needed blood right away. Also excluded were those with a complex antibody profile that made it difficult to match blood.

About 80% of patients received their first transfusion as an inpatient, and 42% of those received it during surgery, the study authors noted.

The researchers collected data on patient characteristics, laboratory and clinical data, and blood bank data from the Ottawa Hospital Data Warehouse. Blood donor data from Canadian Blood Services was linked with hospital data and health administrative data.

The investigators found no statistically significant differences in overall survival between recipients of male donor blood and recipients of female donor blood.

The report was published April 12 in the New England Journal of Medicine.

"Blood is the most common life-saving treatment given in hospital," said Dr. Jason Acker, senior scientist at Canadian Blood Services. "As a blood provider, we were happy to help answer this very important question in transfusion medicine. We hope the findings encourage all eligible donors to continue to donate."

This study was funded by the Canadian Institutes of Health Research.