The rate at which donated hearts are being rejected by transplant centers and surgeons across the United States is increasing despite the growing need for them, says a study.

Without a consistent set of guidelines, donated hearts rejected in one region would be completely usable in another.

In a study published in the American Journal of Transplantation, researchers led by cardiologist Kiran Khush from the Stanford University School of Medicine estimated that over 20,000 patients in the country require heart transplants every year. However, just 1,949 received donated hearts in 2011.

"We've become more conservative over the past 15-20 years in terms of acceptance, which is particularly troubling because of the national shortage of donor hearts and the growing number of critically ill patients awaiting heart transplantation," said Khush.

John Nguyen, a nurse with the California Transplant Donor Network and co-author of the study, explained that a significant number of suitable donor hearts are likely to be unused because of discrepancies in guidelines for determining if a marginal donor heart may be utilized in a transplant. A marginal donor heart is one with undesirable qualities, such as being sourced from an older donor or being small.

According to him, coming up with a more systematic means of evaluating donor hearts using science-based criteria will contribute towards increasing the number of donor hearts accepted for transplants.

With surgeons looking at donor hearts differently, patients end up waiting longer for a transplant, often getting sicker. As such, patients are also often lost during the wait.

For the study, Khush and colleagues looked at data from the Organ Procurement and Transplantation Network involving all potential adult heart donors between 1995 and 2010. They discovered that out of the 82,053 potential hearts donated, just 34 percent were accepted while a bigger portion, 48 percent, were rejected. The remaining 18 percent of donor hearts were used for research and other purposes.

This kind of acceptance rate means that only one out of three donor hearts available is actually being used for transplants. Acceptance rates dropped from 1995 to 2006, going from 44 percent to a meager 29 percent, but slightly bounced back by 2010 with 32 percent.

There are many reasons for rejecting a donor heart but there is also little evidence that waiting for a higher-quality heart will increase chances of survival in a patient or reduce adverse effects.

Acceptance rates are highest in transplant centers northeast of the U.S. while the Pacific Northwest, where there are just a few transplant centers, sees the lowest.

Researchers from Kaiser Northern California, the Victoria University of Wellington and Duke University also participated in the study.