Donor blood can transmit vCJD
Screening donated blood for vCJD is unrealistic and would scare away donors, government advisors say.
Tests for vCJD could be available in 18 months, but all carry a small risk of incorrectly identifying the disease in people who are not actually infected.
The Safety of Blood Tissues and Organs advisory committee says it would consider backing a test that could spot infection with 100% accuracy.
In the UK, four people are thought to have caught vCJD from donor blood.
A number of measures have been put in place to protect people who need transfusions from contracting vCJD.
These include withdrawing any blood products donated from a person who later develops the condition and ruling that no-one who has received a blood transfusion since 1980 can become a donor.
And, since 1998, white blood cells - which are the most likely to carry the infection - have been removed from blood used for transfusions.
However, in 2004, a study of 13,000 appendix and tonsil samples revealed that thousands of people may be unknowingly harbouring vCJD and raising concerns over the possible extent of vCJD transmission via blood transfusions.
Tests to screen potential blood donors for vCJD are in the pipeline, but experts are concerned about their use because some of the time the tests will incorrectly flag up people as infected when actually they are not.
And it is not known whether people with correct positive test results would ever develop symptoms of vCJD, and if they would, how long this would take.
Professor John Forsythe, chair of the advisory committee, said: "If you have a test done and there is a possibility of a false positive of a disease for which there is no treatment at the moment that is very worrying.
"It might put anyone off donating."
He said the UK blood services would legally have to tell donors if their vCJD tests were positive, even if there was a chance that the test result was false.
Professor Mrac Turner, committee member and professor of cellular therapy at Edinburgh University, said research suggested one in every 4,000 people might harbour vCJD in their blood.
However, only a small minority of these will become sick in their lifetime with the disease.
Until a confirmatory blood test for vCJD exists, the advisory committee recommends patients needing blood transfusions should be alerted to the potential risk of transmission and be asked to give written consent for the procedure when possible.
Professor Forsythe said: "There is a risk attached to blood transfusions, albeit small, but I think it is right for patients to know that.
"My drive for saying this is not a legal protection drive. It may be for some others."
A spokesman for the Health Protection Agency said: "Extensive research is being carried out to establish accurate and reliable tests for vCJD. However, the technical challenge is enormous.
"There remains a great deal of uncertainty about this disease, including when infectiousness becomes identifiable in the blood. A rigorous procedure has been designed to evaluate candidate human blood tests."
Between 2.5 and 3 million transfusions and transplants are carried out in the UK every year.
There have been 164 deaths from definite or probably vCJD in the UK since surveillance began in 1990.