Immunoglobulin G Trough Levels and Infection Risk in Adults with Inborn Errors of Immunity Receiving Immunoglobulin Therapy

<i>Background and Objectives:</i> Inborn errors of immunity are increasingly diagnosed in developing countries. Immunoglobulin replacement therapy (IGRT) remains the cornerstone of treatment in these patients, and its monitoring has gained importance for optimizing outcomes. We conducted a retrospective study to evaluate the relationship between IgG trough levels and infections in adults with inborn errors of immunity receiving IGRT. <i>Materials and Methods:</i> Adults diagnosed with primary immunodeficiency and receiving IGRT for at least six months were included. Serum IgG trough levels were measured, and infections during follow-up were systematically recorded. <i>Results:</i> A total of 31 adult patients (CVID: 29, XLA: 2) were analyzed. The mean follow-up was 13 months, with a mean serum IgG trough level of 815.8 ± 205.9 mg/dL, achieved with an average IGRT dose of 498.8 ± 93.0 mg/kg/month. Dose adjustments were made in 35.5% of patients, mostly due to weight changes and clinical indications. Overall, 74.2% of patients had at least one infection requiring antibiotics. Patients with mean IgG trough levels below 850 mg/dL had significantly higher rates of antibiotic-requiring infections (<i>p</i> = 0.032, Mann-Whitney U; mean difference = 0.109, 95% CI: 0.037-0.182; <i>p</i> = 0.005 via <i>t</i>-test). Multivariate regression confirmed that lower IgG trough levels were independently associated with higher antibiotic-requiring infection rates (B = -0.024, 95% CI: -0.045 to -0.002, <i>p</i> = 0.033), while IGRT dose and comorbidities were not significant. <i>Conclusions:</i> IGRT plays a key role in reducing antibiotic-requiring infections in patients with primary immunodeficiency. Regular monitoring and individualized dose adjustments may help optimize outcomes. Further prospective studies are needed to confirm these findings.