Researchers at Cooper University Hospital in Camden, N.J., found that hospitalized patients given transfusions of blood stored 29 days or longer are twice as likely to get a hospital-acquired infection.

The researchers attentively followed 422 patients hospitalized in an intensive care unit who were given blood transfusions from July 2003 to September 2006 and found that whenever they got blood stored 29 days or more, they developed blood stream infections, pneumonia, urinary tract infections, heart valve infections, sepsis and other infections at twice the rate of patients getting blood stored for up to 28 days.

The new research team noted that the existing threshold was established to deal with the fact that while in storage red blood cells release cytokines, which are known to dampen a transfusion patient's immune system thus rendering the patient more susceptible to infection. Cytokine release tends to begin around two weeks into blood storage.

"Stored red blood cells undergo changes that promote the release of a number of biochemical substances called cytokines, which can depress the recipients' immune function and leave them more susceptible to infection," said study author Raquel Nahra. "Those changes start around 14 days of storage."

The FDA usually allows for blood to be stored for up to 42 days before it must be thrown out. Complicating matters, said the researchers, who presented their findings this morning at a conference of the American College of Chest Physicians in Philadelphia, are many institutions' policies of using the oldest available blood first. But shorter blood storage requirements might reduce overall supply.