FRIDAY, Jan. 20 (HealthDay News) -- The first test to help determine the risk of a rare brain infection among users of the drug Tysabri has been approved by the U.S. Food and Drug Administration.

Tysabri (natalizumab) is commonly prescribed to treat multiple sclerosis or Crohn's disease. A relatively small number of users have developed a rare brain infection called progressive multifocal leukoencephalopathy (PML). The newly approved Stratify JCV Antibody ELISA test, combined with other clinical data, can help doctors evaluate the risk of developing PML among people who take Tysabri to treat MS or Crohn's, the FDA said in a news release.

There is "no treatment, prevention or cure for PML, and no certain way to predict who will develop it," the agency stressed. But it said risk factors for PML include the presence of antibodies for the John Cunningham virus, a normally harmless germ except among people with weakened or medically suppressed immune systems.

Other risk factors for developing PML, the FDA said, include taking Tysabri for at least two years, or taking other drugs that suppress the immune system.

The risk for developing PML, believed to be about 11 in 1,000, is greatest among people with all three risk factors, the agency said.

The newly approved test should not be used on its own to measure a person's risk of developing PML, nor should it be used to diagnose the rare infection, the FDA said.

The new test is manufactured by Focus Diagnostics, based in Cypress, Calif. Tysabri is co-marketed by Biogen Idec and Elan Corp.

More information

The FDA has more about Tysabri.