Multiple sclerosis patients using Biogen Idec (Nasdaq:BIIB) drug Tysabri now have access to the first test that can help determine their risk for developing a rare but potentially fatal brain infection that affects some patients using the drug.

The U.S. Food and Drug Administration on Friday cleared the blood test which can help doctors determine whether MS patients as well as Crohn’s disease patients, who are also prescibed the drug, could develop progressive multifocal leukoencephalopathy, or PML. PML has no cure. The risk for the infection means that Tysabri carries a black box warning.

The FDA also granted Weston, Massachusetts-based Biogen Iden a label change for Tysabri that updates the risk factors of developing PML for MS and Crohn’s disease patients.

RBC Capital Markets analyst Michael Yee told Bloomberg News that the label change could push Tysabri’s global sales to $2.5 billion to $3 billion by 2016. Without the label change, sales could top out at $2 billion in 2016.

Biogen makes Tysabri at its manufacturing facility in Research Triangle Park. The drug is marketed with drug partner Elan (NYSE:ELN), who splits revenue with Biogen. Biogen’s share of 2011 Tysabri revenue through the third quarter was $810 million.

The new test measures the presence of antibodies for the John Cunningham virus, or JCV. The FDA said that while the virus is common and generally harmless, people who have weakened immune systems from therapies such as Tysabri have an increased risk of developing PML. Patients at greatest risk of developing PML are those who have been exposed to JCV; have been treated with Tysabri for longer than two years; and are treated with medicines that can weaken their immune systems.

“PML is a fatal infection reported in patients undergoing immunomodulatory therapy,” Alberto Gutierrez, director of the Office of In Vitro Diagnostics in the FDA’s Center for Devices and Radiological Health said in a statement. “This test gives doctors an additional means to determine if MS or CD patients undergoing Tysabri therapy are at increased risk for developing PML.”

The FDA reviewed the JCV antibody test through the de novo reclassification process, a regulatory pathway for novel low- to moderate-risk medical devices that are not comparable to medical devices that are already FDA cleared. The Stratify JCV Antibody ELISA test is manufactured by Focus Diagnostics of Cypress, California.

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