A small study from Johns Hopkins adds to the growing body of
evidence that red blood cells stored longer than three weeks begin to lose the
capacity to deliver oxygen-rich cells where they may be most needed.
In a report published online in the journal Anesthesia & Analgesia, the Johns
Hopkins investigators say red cells in blood stored that long gradually lose
the flexibility required to squeeze through the body's smallest capillaries to
deliver oxygen to tissue. Moreover, they say, that capacity is not regained
after transfusion into patients during or after surgery.
"There's more and more information telling us that the
shelf life of blood may not be six weeks, which is what the blood banks
consider standard," says study leader Steven M. Frank, M.D., an associate
professor of anaesthesiology and critical care medicine at the Johns Hopkins
University School of Medicine. "If I were having surgery tomorrow, I'd
want the freshest blood they could find."
How long should it be
Frank acknowledges that blood banks do not have enough fresh
blood for everybody, and that shorter storage periods would result in
diminished inventory. But he says that the current practice of transfusing
blood stored up to six weeks may need to be reconsidered.
One previous, large study published in the New England Journal of Medicine has
already shown that cardiac surgery patients who received blood stored longer
than three weeks were almost twice as likely to die as patients who got blood
that had been stored for just 10 days.
For the new study, Frank and his colleagues enrolled 16
patients scheduled to have spinal fusion surgery, a type of operation that
typically requires blood transfusions. Six of the patients received five or
more units of blood, while 10 needed three or fewer units.
The researchers drew samples from every bag of blood used —
53 in total — and measured the flexibility of the red blood cells. What they
found is that blood older than 3 weeks was more likely to have less flexible
red blood cell membranes, a condition that may make it more difficult for blood
to deliver oxygen, Frank says.
The team also took blood samples from patients in the three
days following surgery. Even though the blood cells were out of storage and
back in biological environments with proper pH (acidity), electrolytes and
oxygen levels, the injury to the red cells was not reversible and appeared to
be permanent. The damaged blood cells would likely remain dysfunctional for
their life cycle limit, which is up to 120 days, Frank says.
Frank also noted that patients in the study who got fewer
units of blood had healthier red cells overall, even though the blood was just
as old and showed cell damage. He says it is likely that a small amount of
these problem cells make less of a difference than when a large number of
damaged cells are present.
Oldest blood given
According to the research report, the average age of the
blood given in the study was more than 3 weeks. Only three samples in the study
were 2 weeks old or less. One reason for the lack of availability of fresher
bloods for adults, Frank says, is the routine practice of giving paediatric
patients priority for the freshest units.
In fact, he notes, blood banks dispense the oldest blood
first so that it doesn't exceed its shelf life before it can be used. "As
a colleague said, it's like how they sell milk in the grocery store — they put
the oldest cartons out front so they can sell them before they expire,"
Two large randomised controlled studies, one at many centres
across the United States, including Johns Hopkins, and one in Canada, are under
way to determine the relative safety of older versus newer blood, and the
results are expected next year. Frank says blood banks need to be prepared to
change practice if those studies show that a six-week shelf life for blood is
just too long.