LONDON (AP) -- In an effort to reduce the theoretical risk of the SARS virus spreading through blood transfusions and organ transplants, the World Health Organization recommended a variety of special blood safety precautions Thursday.

No severe acute respiratory syndrome patients are thought to have contracted the disease through a contaminated blood transfusion, but WHO officials say they cannot yet rule out the possibility.

The United States and Canada have already tightened blood donation rules to protect against tainting supplies with the SARS bug, but this is the first time the U.N. health agency has made a global recommendation.

"In a case where you don't understand the true risk, you must take maximum measures," said Dr. David Heymann, WHO's chief of communicable diseases. "Guidance with maximum measures is that you don't take blood from people who are in any kind of a convalescent period for this disease in case they might have virus in their blood."

Donors who seem healthy but have cared for, lived with or otherwise had direct contact with a SARS sufferer, or who have recently traveled to a SARS hot zone, should not be allowed to donate blood until three weeks have passed since their trip or last interaction with a SARS patient, the WHO recommends.

In all countries, blood should not be used from probable SARS patients until at least three months after they have fully recovered and stopped treatment. For people suspected to have SARS, the deferral should be one month, WHO says.

Blood from people who show no symptoms and have neither been near a SARS patient nor on a recent trip to areas where SARS is spreading in the community - China, Singapore or the Philippines - should be subject to normal screening, WHO says.

"These recommendations may also be the basis of screening criteria for organs, tissues and cells for transplantation," the guidelines say.

At the moment, there is no reliable test to screen blood donations for SARS. Existing tests that pick up the virus in patients are too unreliable for governments to use for screening national blood supplies, Heymann said.

"What would be important is to make sure they get the right history on all blood donors," Heymann said. "The most appropriate method is the deferral of blood donors, as was done early in AIDS, and is still done in AIDS."

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