Background: Progressive multifocal leukoencephalopathy (PML) remains a concern when considering natalizumab for multiple sclerosis (MS) treatment. Extensive research has identified factors that increase PML risk, and it is important that providers and patients accurately understand risk to make appropriate benefit-risk decisions.

Methods: One hundred adult US patient-candidates for natalizumab therapy were questioned about their PML risk perception, the maximum PML risk they deemed acceptable, and sources of information used to understand risk. Differences in group distributions were compared.

Results: Patients estimated their potential PML risk from 0.1% to 87% (mean, 31.5%). Maximum PML risk deemed acceptable ranged from 0.1% to 45% (mean, 14.5%). Actual risk (mean, 0.01%), based on published risk estimates, was calculated as a function of time receiving therapy, anti–John Cunningham virus antibody index, and previous use of immunosuppressants. The sexes perceived their risks similarly and had similar risk acceptance. Patient perception of PML risk increased with age, whereas willingness to accept risk remained similar among all ages. Higher levels of education correlated with more accurate risk perception and lower risk tolerance. Neither risk perception nor tolerance was correlated with disability level. Sixty-three percent of patients indicated that their primary/referring physician’s concern level regarding potential risk of PML during the benefit-risk discussion was their main source of information about risk.

Conclusions: Patients with MS substantially overestimated their PML risk, often by three orders of magnitude. Patients with MS could benefit from accurate risk education, and providers could play an essential role in presenting PML risk information in a manner understandable to each individual patient.

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