Comparative evaluation of STRATIFY JCV™ and IMMUNOWELL™ assays for anti-JCV antibody detection in natalizumab-treated RRMS patients.

Background: Two principal methods for detecting anti-John Cunningham virus (JCV) antibodies are currently utilized in clinical practice: STRATIFY JCV™, an ELISA developed by Biogen, and IMMUNOWELL™, a solid-phase ELISA assay by Polpharma Biologics/GenBio. Objective: We aimed to evaluate the concordance between STRATIFY and IMMUNOWELL in detecting anti-JCV antibodies in a real-world population of patients with relapsing-remitting multiple sclerosis (RRMS) undergoing natalizumab therapy. Design: This monocentric observational study screened all patients treated with natalizumab for at least 6 months, referring to the MS Center of the University Hospital of Catania. Methods: Each patient's serum was tested simultaneously using STRATIFY-2 (STRATIFY JCV DxSelect) and IMMUNOWELL assays. The qualitative results (positive/negative) were compared, and the index values were analyzed using Pearson's correlation and Bland-Altman plots. Inter-method agreement was calculated using Cohen's kappa coefficient. Results: Among the 120 patients tested, 82 were positive and 31 negative with both STRATIFY and IMMUNOWELL. Four cases were STRATIFY-negative but IMMUNOWELL-positive, and three were the opposite. Overall concordance was 94.2%, with a Cohen's Kappa of 0.86, indicating strong agreement. The index values showed strong correlation (Pearson r = 0.79, p < 0.001) and the coefficient of determination (r 2) was 0.62. Conclusion: STRATIFY and IMMUNOWELL demonstrate a high level of agreement in the detection of anti-JCV antibodies in patients with RRMS receiving natalizumab. IMMUNOWELL may serve as a reliable complementary method, especially in cases where borderline serostatus could influence therapeutic strategy. Regular and accurate monitoring of JCV status remains essential for guiding long-term treatment safety and optimizing individual patient outcomes.