From Low-Positive Fixed Cell-Based Assay to Confirmed Myelin Oligodendrocyte Glycoprotein Antibody-Associated Disease: A Paediatric Case Report

Background: The diagnosis of myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) relies on sensitive serological detection of MOG-IgG. Fixed cell-based assays (CBAs) may yield low-positive or borderline results that complicate early clinical decision-making, whereas live CBAs—recommended as the reference method—preserve native antigen conformation and offer higher analytical sensitivity. Importantly, low-positive titres should not be confused with true seronegativity, as they may nevertheless be clinically meaningful. Case Presentation: A 14-year-old previously healthy male presented with left optic neuritis and perineuritis following an upper respiratory infection. Initial MOG-IgG testing on a fixed CBA was low-positive (1:10). He partially responded to intravenous methylprednisolone and required intravenous immunoglobulin (IVIG) for complete resolution. Over three years, he experienced sequential, steroid-dependent bilateral optic neuritis with perineuritis, relapsing on every steroid taper. Rituximab and subsequently mycophenolate mofetil failed to induce remission. Repeat testing with a live CBA at a reference laboratory yielded a high-positive MOG-IgG titre of 1:1000, confirming MOGAD. Tocilizumab (8 mg/kg every 4 weeks) was initiated and allowed complete corticosteroid withdrawal. At age 18, the patient remained asymptomatic, with an Expanded Disability Status Scale score of 0, best-corrected visual acuity of 20/20 in both eyes, and stable peripapillary retinal nerve fibre layer thickness on spectral-domain optical coherence tomography. Conclusions: In paediatric patients with recurrent optic neuritis with perineural involvement and borderline fixed-CBA results, confirmatory testing with a live CBA at a reference laboratory should be considered to avoid diagnostic delay and therapeutic misdirection. In refractory, steroid-dependent cases, IL-6 receptor blockade may represent a reasonable therapeutic option, in line with emerging evidence.