Anti-JCV antibody seroprevalence and testing practices in multiple sclerosis patients considered for natalizumab...

Progressive multifocal leukoencephalopathy (PML) is a feared complication of natalizumab therapy, and anti-John Cunningham virus (JCV) antibody serology is central to risk stratification. Data on JCV seroprevalence and testing practices in Saudi Arabia remain limited.

To determine the prevalence of anti-JCV seropositivity in a cohort of patients with multiple sclerosis (pwMS), assess the proportion tested prior to natalizumab initiation, and evaluate laboratory turnaround time.

A cross-sectional observational study at King Saud Medical City, Riyadh, Saudi Arabia, including all pwMS diagnosed according to the 2017 revised McDonald criteria who underwent at least one anti-JCV antibody test between January 2019 and December 2024. Follow-up data were truncated on February 18, 2025. Testing was performed using the STRATIFY JCV assay (Biogen/Unilabs).

Ninety-one patients were included; 68 (74.7%) were female, with median age of 28.8 years (IQR 24.4-34.8). Anti-JCV seropositivity was identified in 36 patients (39.6%); of these, 4 (11.1%) had an index value ≤0.9, 8 (22.2%) had a value >0.9-1.5, and the majority exceeded 1.5 (24 patients, 66.7%). Eighty-three patients (91.2%) received natalizumab; of these, only 4 (4.8%) were tested prior to initiation, and none had their result available before the first infusion. Among seropositive patients, 32 (88.9%) initiated natalizumab, with a median treatment duration of 12.1 months (IQR 3.9-24.4). The median laboratory turnaround time was 20 days (IQR 13-27).

JCV seropositivity was identified in approximately 39.6% of MS patients considered for natalizumab therapy at our center, consistent with regional estimates. Pre-treatment JCV testing was largely absent, and turnaround time was prolonged.