Residual disease activity in cystic fibrosis lung disease: implications for clinical management and research.

The advent of cystic fibrosis transmembrane conductance regulator (CFTR) modulators is fundamentally reshaping research and care for people with cystic fibrosis (CF). Despite unprecedented improvements in lung function and predicted survival, a substantial proportion of people with CF continue to exhibit residual disease activity (RDA) across multiple biological and clinical domains. Persistent abnormalities in CFTR function, mucus properties, mucociliary clearance, airway infection, inflammation, and symptom burden indicate that CFTR modulation does not fully normalize airway physiology or eliminate the pathological processes underlying CF lung disease. This review proposes RDA as a conceptual framework for identifying ongoing, potentially modifiable disease processes in people receiving CFTR modulators. Candidate measures across complementary mechanistic and clinical domains are examined, and their potential multidimensional interpretation as a means of supporting more individualized monitoring and treatment decisions is discussed. We consider the implications of RDA for clinical research by identifying key evidence gaps, outlining a stepwise pathway for its definition and validation. We further explore the potential to inform sensitive endpoint selection, enrich study populations, and guide the development of therapies targeting disease processes that persist despite CFTR modulation. As the CF population ages and disease trajectories evolve, care models and research priorities must move beyond conventional measures of disease severity alone. A better understanding and systematic evaluation of RDA may help align long-term clinical management and future trial design with the changing needs of people with CF, while ensuring that therapeutic advances translate into sustained health gains.