“Low‐Positive” MOG‐IgG Cases Among Adults With a First Event Suggestive of Multiple Sclerosis

Abstract

ABSTRACT Objective To determine the prevalence and clinical characteristics of patients with “low‐positive” (LP) MOG‐IgG (titres 1:160–1:320) among adults with a first demyelinating event (FDE) suggestive of multiple sclerosis (MS). Methods From the Barcelona CIS inception cohort, we included adult patients with serum collected ≤ 6 months from the FDE. MOG‐IgG was assessed by a cell‐based assay with flow cytometry (CBA‐FC). Demographic, clinical, and paraclinical data were compared among seronegative, LP, and “clear‐positive” (CP; ≥ 1:640) patients. Supporting MOGAD features and final diagnoses were retrospectively reviewed in seropositive cases. Results Of 613 patients, 42 (6.9%) were MOG‐IgG positive (CP = 17; LP = 25). LP patients were indistinguishable from seronegative patients but differed from CP patients, who more frequently had optic neuritis, lacked cerebrospinal fluid‐oligoclonal bands, and were less likely to meet the McDonald criteria (p < 0.05). At the last follow‐up, 64% of LP versus 18% of CP patients were diagnosed with MS (p = 0.004). Only one LP patient fulfilled MOGAD criteria, compared with 14 CP patients (p < 0.001). Interpretation Lowering the CBA‐FC positivity threshold to ≥ 1:160 had limited diagnostic yield in this MS‐predominant cohort, as most LP cases were ultimately diagnosed with MS. These findings support cautious interpretation of LP MOG‐IgG results in this clinical setting.