Elevated Immunoglobulin G as a Predictor of Progression to Severe Lung Disease in Cystic Fibrosis: A Longitudinal Cohort Study

Background: Elevated immunoglobulin G (IgG) levels are associated with worse lung function and disease severity in people with cystic fibrosis (PwCF). This study evaluated whether elevated IgG levels—defined as values above the 97.5th percentile (Z-score ≥ 1.96 standard deviations above the mean)—can predict progression to severe lung disease. Methods: A retrospective cohort study of children and adults with CF at a single-center clinic was performed. Patients with elevated baseline IgG Z-scores were compared to those with normal or low IgG levels. Progression to severe lung disease was defined as % predicted FEV₁ < 40%, referral for lung transplantation, or death. Kaplan–Meier survival curves and Cox models were used to analyze clinical outcomes. A sensitivity analysis was conducted for patients aged 18 years or older. Results: Of 97 patients, 31 (31.9%) had elevated IgG levels. Progression to severe lung disease occurred in 14 (14.4%) patients, 12 (85.7%) of whom had elevated IgG. These patients were significantly older and had a higher prevalence of Pseudomonas aeruginosa colonization. Among adults, those with elevated IgG had lower baseline % predicted FEV1 and greater annual lung function decline. Elevated IgG was independently associated with progression to severe lung disease (adjusted hazard ratio [aHR]: 9.8; 95% CI: 1.9–48.6), even after adjusting for Pseudomonas colonization and annual % predicted FEV1 decline. Conclusions: Elevated IgG was associated with progression to severe lung disease in PwCF and correlated with older age, Pseudomonas colonization, and—in adults—lower baseline lung function and faster decline. These findings highlight elevated serum IgG as a meaningful prognostic biomarker for identifying high-risk PwCF who may benefit from closer monitoring and earlier intervention.