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Biogen Idec Inc.'s diagnostic test that may help determine the risk of developing a rare brain infection in patients using multiple sclerosis drug Tysabri has received marketing approval in Europe.

Tysabri, sold with Elan Corp, is a highly effective treatment for the debilitating disease, but it has been linked in a small number of patients to progressive multifocal leukoencephalopathy, or PML. Global sales rose 16% to $ 1.2 billion last year, although growth has been lower than originally expected in recent years because of the risks of the infection....Read More - http://www.msrc.co.uk/index.cfm/fuseact ... ageid/1905

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Reading about this test on the net indicates that having a negative result doesn't guarantee you won't get PML. Having a positive test doesn't mean you will necessarily get PML. Some patients don't want to have the test and are willing to take the chance they will be OK.
Some patients who get a positive test still want to try Tysabri but their docs won't prescribe it for them under the circumstances.

Has to be frustrating for all involved.

Harry