Use of older blood also is associated with an increased risk for kidney problems, acute respiratory distress and longer confinement to intensive care units, the researchers found.
The findings may hold important health implications, but they should be viewed with caution, the researchers said.
"We did find a significant association between older blood and adverse outcomes," said Elliott Bennett-Guerrero, M.D., a Duke anesthesiologist who was the study's lead investigator. "However, there have been no large, randomized clinical trials conducted to uncover links between age of transfused blood and patient outcomes, so we cannot say for certain that older blood causes the adverse effects we found in our study.
"Our results show that the number of transfusions given is a robust predictor of long-term mortality, and that the duration of storage also has independent adverse effects as well," he continued. "We believe our findings provide intriguing avenues for future study."
The researchers published their findings on June 22, 2006, in the journal Anesthesia & Analgesia. The study was supported by Duke University Medical Center and the Columbia University College of Physicians & Surgeons.
Patients routinely receive transfusions during and after surgery to replace lost blood. Of the 12 million units of blood administered to patients each year in the United States, more than 2 million units are transfused into heart surgery patients.
National blood banks require that blood can be stored for only 42 days after donation. After that time, unused blood must be discarded.