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The wait for a heart transplant varies widely based on factors such as availability of donor hearts and blood type, but little is known about differences in wait times based on race and ethnicity. Now, preliminary research suggests African-American patients may experience longer wait times than other racial and ethnic groups.

Who gets a donor heart each time one becomes available is based on objective criteria such as blood type, body size and how urgently a patient needs a transplant. Wait times, which are partially driven by the geographic availability of donor organs, are generally months long.

In the study, presented Saturday at the 91ÊÓƵ's Scientific Sessions conference, researchers looked at people awaiting their first heart transplant between 2005 and 2016. Of the 36,801 patients, about 67 percent were white, 22 percent were African-American and 8 percent were Hispanic.

The wait time peaked in 2014 for patients across all racial and ethnic groups but was disproportionately longer for African-American patients – a median of 19.8 months compared with 12 months for white patients and 12.3 months for Hispanic patients. The disparity was less pronounced in the years that followed. In 2016, for example, median wait time was 10.1 months for African-American patients, eight months for white patients, and 7.4 months for Hispanic patients.

"After adjusting for some of the variables that could impact wait time like age, blood type and geographical region, the time to transplant was observed to be longer for African-Americans compared to whites," said Dr. Anuradha Lala, lead author of the study and an assistant professor at Mount Sinai Medical School in New York.

Lala cautioned that it's too early to draw definite conclusions about what caused the difference in wait time. The findings, she said, did not account for other factors that may have had an impact.

"It is important to understand whether disparities exist and, if so, the next step is to understand the factors that underlie the differences in wait times and, of those factors, which can we target for intervention to reduce or eliminate disparities," she said.

However, wait times and donor supply are dynamic and can change rapidly, and some experts are skeptical that the differences are due to race.

"I don't think we can infer a high degree of bias in the decision (of heart allocation). So much is driven by objective measures," said Dr. Clyde Yancy, chief of cardiology and Magerstadt Professor of Medicine at Northwestern University's Feinberg School of Medicine in Chicago.

"It's impossible to control for all other variables and have race and ethnicity emerge as the predominant reason for longer wait times. You're dealing with people at the edge of life with a number of comorbidities and variables that determine their clinical stability or instability," said Yancy, who was not involved in the study. "We will use those hearts in the very best way we can, for those who are desperately ill and have a chance of surviving."

Lala believes this is an important area for ongoing investigation, especially in light of the United Network for Organ Sharing's new heart allocation system that took effect last month. The new system is designed to ensure that the most critically ill patients receive a transplant sooner. Candidates are now listed according to six medical urgency statuses and there's more flexibility in terms of how far a heart can travel from donor to recipient.

But it's unclear how the new system will ultimately impact wait times and outcomes.

"We need to continually look at wait times under the new system," Lala said. "If we notice there are disparities, we need to try to understand why and what can be done to minimize them."

Find more news from Scientific Sessions.

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