Natalizumab, a humanized monoclonal antibody targeting α4-integrin, is an effective therapy for relapsing–remitting multiple sclerosis (RRMS). However, its use carries a risk of progressive multifocal leukoencephalopathy (PML), a rare but potentially fatal brain infection caused by John Cunningham virus (JCV) reactivation. Robust risk stratification is therefore essential to ensure patient safety.

The cornerstone of this strategy is detection of anti-JCV IgG antibodies in serum or plasma. Antibody presence is the most significant risk factor for developing PML in natalizumab-treated patients, alongside prior immunosuppressant use and treatment duration exceeding two years. The STRATIFYJCV™ DxSelect™ assay (Focus Diagnostics, Cypress, CA, USA) has been the principal clinical assay for over a decade, providing quantitative antibody index values incorporated into widely used PML risk stratification algorithms and guiding clinical decisions through quantitative index values that stratify patients into risk categories (Ho et al., 2017).

The recent introduction of natalizumab biosimilar aims to reduce healthcare costs and broaden access to this effective therapy (Hemmer et al., 2023). However, it has been accompanied by the implementation of an alternative anti-JCV antibody assay, ImmunoWELL™ JCV IgG. Recent studies highlight significant variability between assays, which may alter patient classification and complicate risk assessment (Ziemssen and Centonze, 2025).

In our institution, the natalizumab biosimilar was introduced together with its companion ImmunoWELL™ JCV IgG assay (GenBio, San Diego, CA, USA) in March 2024. Soon after its implementation, an unexpectedly high rate of JCV seropositivity was observed, which led to the design of the present study.

The objective of this study was to evaluate inter-assay concordance between STRATIFYJCV™ DxSelect™ and ImmunoWELL™ JCV IgG in paired samples from natalizumab-treated patients, quantifying both magnitude and directionality of discordance and exploring their clinical relevance across different risk thresholds.