Safety Monitoring for Tysabri (Natalizumab)
For patients on Tysabri, safety lab monitoring frequency depends critically on JC virus (JCV) antibody status and index value, with JCV antibody testing required every 6 months for most patients, and MRI surveillance frequency ranging from annually to every 3-4 months based on PML risk stratification. 1
Initial Baseline Assessment
Before or around the time of initiating Tysabri therapy, obtain:
- Baseline JCV antibody testing with index value to establish PML risk stratification 1
- Baseline brain MRI (T2, DWI, FLAIR sequences minimum) to serve as reference for future safety monitoring 1
During the first 12 months of therapy, no additional safety monitoring beyond baseline is required, as PML risk is extremely low (1 in 10,000 to 1 in 1,000) regardless of JCV status 1
Ongoing Safety Lab Monitoring (After 12 Months)
JCV Antibody-Negative Patients
- Retest JCV antibody status every 6 months to detect seroconversion 1, 2
- PML risk remains approximately 1 in 10,000 1
- Annual MRI surveillance is sufficient 1
JCV Antibody-Positive Patients with Index ≤1.5
- Retest JCV antibody index every 6 months to monitor for index increases 1, 2
- PML risk approximately 1 in 5,882 1
- MRI surveillance every 6 months minimum after 18 months of treatment 1, 2
JCV Antibody-Positive Patients with Index >1.5
- Further JCV antibody index testing is not mandatory (risk already established as high) 1
- PML risk approximately 1 in 855 overall, increasing to 1 in 113 during months 25-48 of therapy 1
- MRI surveillance every 3-4 months after 18 months of treatment 1, 2
MRI Safety Monitoring Protocol
All safety MRIs should include:
- Minimum sequences: T2, DWI, and FLAIR of the brain 1
- Performed and reported by neuroradiologist or radiologist trained in PML identification 1
- Mandatory MRI at 12 months for all patients regardless of JCV status 1
- Consider full protocol including cervical and thoracic spine annually 1
- Gadolinium enhancement if clinical disease activity increases 1
Critical Monitoring Considerations
Clinical vigilance cannot be replaced by testing schedules - any new neurological symptoms warrant immediate evaluation regardless of scheduled monitoring intervals 1, 3
MRI evidence of PML may appear 3-6 months before symptom onset, making adherence to imaging schedules critical for early detection 1
Patients with prior immunosuppressant use and JCV-positive status beyond 2 years of therapy should default to 3-4 monthly MRI frequency 1
The risk of PML increases significantly beyond 24 months of treatment duration, particularly in JCV-positive patients 4, 5