PRIORITY Study: Baseline and Post-Rituximab Immune Profiles in a Single Centre

Background Pediatric utilization of rituximab encompasses autoimmune, renal, hematologic, and neurologic conditions. Its association with secondary hypogammaglobulinemia and unmasking of inborn errors of immunity is well recognized. However, incomplete pretreatment immune evaluation may lead to misattribution of preexisting immune abnormalities to rituximab or delayed recognition of underlying immune disorders. We conducted a quality improvement initiative to assess immune monitoring practices surrounding rituximab use at our institution. Objectives To evaluate the baseline frequency of pre- and post-rituximab immune testing and describe the prevalence of hypogammaglobulinemia before and after rituximab therapy using age-adjusted pediatric reference ranges. This was to identify potential care gaps and targets for quality improvement. Methods We conducted a retrospective chart review of pediatric patients treated with rituximab at McMaster Children’s Hospital between 2022 and 2025. Extracted data included demographics, rituximab dosing, immunoglobulin levels (IgG, IgA, IgM, and IgE), and lymphocyte subsets (CD3, CD4, CD8, and natural killer [NK]) when available. Values were interpreted using age-specific reference ranges. Descriptive statistics were performed. Results Twenty-seven patients were included. Pre-rituximab IgG, IgA, and IgM levels were available in only 15/27 patients (55.6%). Post-rituximab IgG, IgA, and IgM levels were available in 16/27 patients (59.3%), with post-rituximab immunoglobulin measurements obtained at least one year after the final rituximab dose. Baseline lymphocyte subset analysis (CD3, CD4, CD8, and NK percentages) was available in 7/27 patients (25.9%), increasing to 13/27 (48.1%) post-rituximab. Among patients with available baseline IgG data, 6/15 (40.0%) had IgG levels below age-adjusted reference ranges prior to rituximab initiation, representing 22.2% of the total cohort. Post-rituximab, 9/16 patients (56.2%) had low IgG levels, representing 33.3% overall. Low baseline IgA was observed in 7/15 patients (46.7%), while low baseline IgM was uncommon, 1/15 (6.7%). Conclusions More than half of the children receiving rituximab lacked a complete baseline immune evaluation. A substantial proportion had hypogammaglobulinemia prior to therapy, particularly affecting IgG and IgA. These findings highlight gaps in immune monitoring and the risk of misattributing preexisting immune abnormalities to rituximab. Standardizing pre- and post-treatment immune testing across specialties is needed to improve the identification of treatment-associated immune complications and underlying inborn errors of immunity.