Answer
It means previous exposure to JC virus and a higher PML risk than a negative result. It does not mean that PML is present; the antibody index and other risk factors guide monitoring and treatment discussions.
Sources for this answer
- Tysabri 150 mg solution for injection in pre-filled syringe — Current selected SmPCemc · Summary of Product Characteristics
- TYSABRI 300 mg concentrate for solution for infusion — Current selected SmPCemc · Summary of Product Characteristics