Reassessing the risk of natalizumab-associated PML.

The risk algorithm for natalizumab-associated PML was first established in 2012 using the observations that JC virus antibody status, prolonged duration of natalizumab therapy (>2 years), and prior exposure to immunosuppressive therapy increased the risk for the disease. Prior to the publication of Biogen's algorithm, a risk algorithm was created by Fox and Rudick using an Excel spreadsheet in order to address the concerns of their patients. Applying the most recently available data regarding natalizumab-associated PML, the risk assessments for PML were recalculated. The current numbers indicate substantially higher risks for PML in 2015 than in 2012. Our calculations suggest that an individual having all three risk factors has an approximately 1 in 44 chance of developing PML.