Storing donated blood too long increases the chance of an infection, US researchers claim.
The risk of blood poisoning or pneumonia doubled once the 29-day mark passed, Cooper University Hospital in New Jersey found.
The study, presented at a US conference, calls for increased care over the way blood is used and stored.
UK authorities said blood took 10 days on average to reach hospitals and daily deliveries meant it was not stockpiled.
The US does not allow the use of blood stored for longer than 42 days – in the UK this is lower, at 35 days.
After two weeks in storage, red blood cells start to undergo changes which lead to the release of chemicals called “cytokines”.
These are known to hinder immune function, and in high levels could possibly make patients more susceptible to infection.
Researchers looked at the rate of hospital infections in 422 patients against the age of the blood transfusion they received.
They found that the average age of the blood was 26 days, and 70 percent of patients had received blood older than 21 days.
In total, 57 patients developed an infection – and these patients had received older blood than the others – on average it had been stored for three and a half days longer.
Patients who had received blood older than 28 days were twice as likely to develop an infection, and the more units of blood given, the higher was the chance of infection.