Evaluation of Dried Plasma Spot‐Based Quantification of Glial Fibrillary Acidic Protein as a Disease‐Associated Biomarker in Neuromyelitis Optica Spectrum Disorder

Objective To evaluate the diagnostic accuracy of glial fibrillary acidic protein (GFAP) measured in dried plasma spots versus conventional plasma‐ and serum‐GFAP testing for assessment of disease severity in aquaporin‐4 immunoglobulin G–positive neuromyelitis optica spectrum disorder (AQP4‐IgG + NMOSD). Methods A neuroimmunological prospective cohort of remission samples from 70 participants with the diagnoses AQP4‐IgG + NMOSD ( n = 19), myelin oligodendrocyte glycoprotein antibody–associated disease (MOGAD; n = 9), relapsing–remitting multiple sclerosis (RRMS; n = 28) and healthy controls (HC; n = 14) from a single center were included. GFAP concentrations were measured in frozen and thawed plasma and paired dried plasma spot (DPS) samples using an ultrasensitive proximity ligation–based assay (nucleic acid–linked immuno‐sandwich assay; NULISA). In NMOSD, GFAP was additionally quantified in paired serum samples using a single‐molecule array (Simoa) assay as a reference. Cross‐matrix correlations, diagnostic performance, and associations with neurological disability were evaluated. Results GFAP concentrations measured by NULISA correlated strongly between plasma and DPS samples across diagnostic groups and healthy controls. In NMOSD, plasma GFAP measured by NULISA showed strong concordance with serum GFAP quantified by Simoa. DPS‐derived and plasma‐derived GFAP demonstrated good diagnostic accuracy for AQP4‐IgG + NMOSD and was significantly associated with neurological disability as measured by the Expanded Disability Status Scale (EDSS). Group‐wise comparisons across plasma and DPS showed retained elevation of GFAP in AQP4‐IgG + NMOSD compared with the other diagnostic groups and healthy controls. Interpretation GFAP quantification using the NULISA platform is feasible in plasma and DPS samples and enables reliable biomarker assessment in a potentially remote‐compatible setting. DPS‐derived GFAP measurements retained meaningful information on disability in NMOSD and may provide an analytical framework for future studies evaluating minimally invasive capillary or self‐sampling approaches.