Threshold-Dependent Discordance Between STRATIFY JCV™ and IMMUNOWELL™ Assays in Relapsing Multiple Sclerosis

Introduction: Anti-JC virus (JCV) antibody testing is central to progressive multifocal leukoencephalopathy (PML) risk stratification in patients treated with natalizumab. While STRATIFY JCV™ is the historical reference assay, IMMUNOWELL™ is increasingly used in the context of biosimilar adoption. We aimed to determine whether inter-assay discordance reflects true biological divergence or threshold-dependent interpretative effects within clinically relevant PML risk categories. Methods: In this single-center observational study, 250 consecutive patients with relapsing multiple sclerosis (RMS) underwent same-day paired STRATIFY and IMMUNOWELL testing. Agreement was evaluated under two interpretative frameworks: (i) manufacturer-defined analytical cutoffs and (ii) clinically adopted PML risk-based thresholds. Quantitative comparability was assessed using Pearson correlation and Bland–Altman analysis. Results: Using analytical cutoffs, concordance was 78.0% (κ = 0.55), with discordance predominantly in borderline index ranges. When applying clinically adopted risk thresholds, concordance increased to 93.2% (κ = 0.73). Index values correlated strongly (r = 0.922; p < 0.0001). Bland–Altman analysis showed minimal mean bias (+0.06), with wider deviations confined largely to low-to-intermediate index values rather than high-risk profiles. Conclusions: Inter-assay discordance between STRATIFY and IMMUNOWELL is largely threshold-driven and substantially attenuated when applying clinically meaningful PML risk categories. These findings support practical interchangeability for routine monitoring in the biosimilar era, while underscoring the need for cautious interpretation of borderline results.