The Food and Drug Administration has approved the first blood test to screen blood, organ and tissue donors for the West Nile virus.

But experts say the test takes so long to process that it might not provide much additional help in preventing the virus from being transmitted through organ donation, as occurred earlier this year in several patients, including a man who received a lung in Pittsburgh from an infected New York donor.

FDA announced approval of the Procleix WNV Assay, developed by Gen-Probe Inc. and marketed by Chiron Corp., late last week.

The approval "is one step in making the donor pool safer for blood and organ donation. But there still needs to be enhancement in the technology," said Dr. Jay Fishman, an associate professor at Harvard Medical School.

Available tests, including the one just approved by the FDA, can take hours to process, which can be too long in many cases for use in organ donors, said Dr. Fishman, who directs the transplant infectious disease program at Massachusetts General Hospital. He said other tests are being developed that might yield results more quickly.

Susan Stuart, president of the Center for Organ Recovery and Education, the Pittsburgh region's organ procurement organization, said yesterday that she did not expect major changes to result from approval of the test.

Blood donor centers already screen for the West Nile virus using tests approved by the FDA for investigational use, Ms. Stuart said. Organ donors are not routinely screened for the virus, though they are screened for other conditions such as hepatitis and HIV.

People who contract the West Nile virus usually have only mild symptoms, but the virus can cause life-threatening inflammation of the brain or spinal cord, according to the National Institutes of Health. Most cases of the disease occur in elderly people and those with impaired immune systems.

In rare cases, West Nile infection has been reported in organ transplant patients.

Earlier this year, a 69-year-old New York man who received a lung transplant at the University of Pittsburgh Medical Center was among three transplant recipients who apparently contracted West Nile virus from a organ donor who died in late August.

The lung transplant recipient, who had been in critical condition two months ago, is now in serious condition, UPMC said yesterday.

The other two patients received liver and kidney transplants in New York City.

The cases were just the second time that West Nile virus infection was reported in the United States in connection with organ transplants; one patient died in the first incident in 2002.

The FDA said there have been 30 cases where people most likely acquired West Nile virus from blood transfusions, and nine of those patients died.