WASHINGTON (Reuters) - Patients given transfusions of blood stored 29 days or longer — well within US standards — are twice as likely to get a hospital-acquired infection as those getting newer blood, researchers said on Tuesday.

US Food and Drug Administration regulations allow for blood to be stored up to 42 days before it must be discarded.

But researchers at Cooper University Hospital in Camden, New Jersey, found that blood stored nearly two weeks less than that still might be problematic.

The researchers tracked 422 patients hospitalized in an intensive care unit who were given blood transfusions from July 2003 to September 2006.

If 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 infections were not caused by blood that was already tainted at the time it was donated, but were caused because it degraded over time, the researchers said.

Stored red blood cells experience changes that promote the release of biochemical substances called cytokines that can lower a patient's immune function and render them more vulnerable to infection, the researchers said.

"We're not talking about hepatitis, HIV or other things that are transmitted in the transfused blood, but an increased susceptibility to infection as a result of the transfusion," Dr. David Gerber of Cooper University Hospital, one of the researchers, said in a telephone interview.

"There are significant policy implications for this. Transfusion is still an important medical practice," added Gerber, whose findings were presented at a meeting of the American College of Chest Physicians.

The average age of blood used in US transfusions is around 17 days, the researchers said.

Gerber did not endorse shortening the 42-day policy, saying that could cut the amount available in the blood supply.

"We live in a complex world here. There is a finite supply of donated blood," Gerber said. "In the back of my mind I always have to wonder what happens if there is a natural disaster or an unnatural disaster, and there's a tremendous strain on the blood supply."

Dr. Richard Benjamin, chief medical officer for the American Red Cross, said the study shows the need for more data on the effect of the age of blood on patient outcomes.

"However, the judicious use of blood transfusion allows medical and surgical procedures that would not otherwise be possible due to blood loss. Physicians and patients need to weigh the potential benefits against the small risk of harm caused by transfusion," Benjamin said in a statement.

The study is not the first to find health problems in people getting transfusions of relatively older blood.

Researchers at the Cleveland Clinic in Ohio reported in March that heart surgery patients who received transfusions with blood stored more than 14 days were more likely to experience complications including shorter survival times.