Biogen Idec has reported promising early data for a test to help determine which patients taking its multiple sclerosis drug, Tysabri, are at greatest risk for developing a rare but serious brain infection.

Biogen and its partner, Ireland-based Elan Corp. plc announced that data has been published in the Annals of Neurology on an assay to detect antibodies to a virus associated with the brain infection, known as progressive multifocal leukoencephalopathy.

The goal is to develop a test that could be used to determine which patients might be most at risk for developing PML, to alert patients and doctors to potential risks, and improve clinical management of MS patients.

“These data support our ongoing clinical studies to assess the clinical utility of this assay and our commitment to further mitigating the rare risk of PML in TYSABRI-treated patients,” said Dr. Alfred Sandrock, senior vice president of neurology research and development at Biogen Idec, in a statement.

More than 60,000 patients have taken Tysabri since the drug’s reintroduction in July 2006. The drug had previously been taken off the market between February 2005 and July 2006 because of concerns about the risk of PML. Tysabri’s warning labels said the anticipated incidence rate of PML is one in 1,000, and Biogen said the current rate is still below that.

The number of cases is rising because the longer patients take the drug, the greater the risk of developing the infection, Biogen officials have said.

Tysabri sales reached $1 billion in 2009.