Apparent per-session removal of AQP4-IgG during therapeutic plasma exchange in a patient with aquaporin-4 antibody–positive neuromyelitis optica spectrum disorders: a case report

Therapeutic plasma exchange (TPE) is an established treatment for neuromyelitis optica spectrum disorders (NMOSD) that removes pathogenic aquaporin-4 antibodies (AQP4-IgG). We present the case of a 15-year-old boy with AQP4-IgG–positive NMOSD who underwent six TPE sessions. Quantitative analysis revealed mean removal rates of 84% for total immunoglobulin G (IgG) and 49% for AQP4-IgG. The calculated apparent volume of distribution for AQP4-IgG was greater than that of total IgG in this one-compartment model. This finding may be consistent with more complex distribution behavior than that of total IgG, but mechanistic interpretation is limited by the absence of serial sampling. These results suggest that disease-specific autoantibodies may exhibit lower apparent removal efficiency than total IgG during TPE, which may have implications for clinical management in NMOSD.