Infections are not the whole story, however.
Within hours of being collected, red cells become stiff, making them less able to squeeze into narrow capillaries - essential if they are to deliver oxygen to organs. The changes are triggered in part by white cells, although it is not known how they might do this. Blood banks in the UK routinely filter blood to remove any white cells, something which is not done everywhere in the US or Australia.
Chemical changes also take place that limit the ability of red cells to deliver oxygen to the tissues. For example, levels of nitric oxide (NO), which signals blood vessels to open, drop dramatically within a day of collection. "We are now working on the best way to put NO back into blood on a large scale," says Jonathan Stamler of Duke University in North Carolina.
Another study, published in March, suggests the longer red cells are stored, the poorer their quality. It found patients who received blood more than two weeks old were almost 70 per cent more likely to die within a year than those who got newer blood.
"If all blood had to be used within two weeks, it would cause a major inventory problem," says Isbister, adding that the finding highlights the need to look for better ways to store blood. Just as important is the need for clinical trials to work out who benefits from transfusions and who doesn't. "We need 60 or 70 randomised clinical trials right now," says Spiess.
But people should not stop donating blood, stress experts. "Transfusion is critical in several situations such as severe haemorrhage. We also need blood for essential products such as antibodies and clotting factors for people with haemophilia," says Isbister.
Bloodless surgery
"Reduce, reuse, recycle" is usually a mantra for the environment, but it applies to "bloodless surgery" too.
It was originally developed to enable Jehovah's Witnesses, who shun blood transfusions, to undergo major surgery and as long as surgeons use special techniques they can undergo operations that have the greatest potential for blood loss, such as open-heart surgery - without ever receiving a drop of someone else's blood.
But as safety concerns have spread so has its use. It may involve little more than treating any anaemia prior to surgery, reducing the blood taken for tests, and meticulous surgery.
"Most general surgery patients who receive a transfusion get one or two units of blood. With careful surgery you can avoid losing that amount in the first place," says Nicolas Jabbour at the Baptist Medical Center in Oklahoma City.
Special techniques can also be used.For example, at the New Jersey Institute for the Advancement of Bloodless Medicine and Surgery at Englewood Hospital, patients who have lost a lot of blood may spend time in a hyperbaric chamber after surgery in an attempt to load their remaining red cells with oxygen.
More commonly, during or after surgery, spilt blood is collected, cleaned and reinfused. The process has the disadvantage that it removes proteins that stimulate clotting. An alternative is to remove some blood before surgery and replace it with saline or another fluid. After surgery, the patient's blood is returned.
Bloodless surgery does work, suggests a 2006 study comparing 49 Jehovah's Witnesses and 196 non-Jehovah's Witnesses who underwent cardiac surgery, which found comparable death rates during surgery.
From issue 2653 of New Scientist magazine, 26 April 2008, page 8-9
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