Childhood Interstitial Lung Disease—Successful Application of a Stepwise Diagnostic Classification

Background/Objectives: Childhood interstitial lung disease (chILD) represents a heterogeneous group of rare pulmonary disorders. Practical diagnostic approaches tested for feasibility and impact in comprehensive cohorts are lacking. We aimed to assess a simple etiologically focused classification approach, clarify the role of genetic testing and quantify the impact of non-pulmonary organ manifestations. Methods: We hypothesized that chILD can be classified in a clinically meaningful and versatile way by answering three questions: Which children have an etiological chILD diagnosis due to (1) identified (exposure-related) cause/lung injury, or (2) systemic disease? (3) In how many children without an etiological diagnosis can a genetic cause be identified? We also calculated the predictive value of non-pulmonary organ involvement for underlying systemic conditions. Results: Among 1693 patients, 24.7% were grouped as ILD related to exposure, 22.7% as ILD with systemic condition, 16.6% as ILD with genetic diagnosis of systemic disease, 10.0% as ILD with genetic diagnosis affecting the lungs only, and 25.8% as ILD without genetic diagnosis. The average genetic diagnostic yield was 50.8%, with higher rates in interstitial pneumonia (61.4%) or pulmonary alveolar proteinosis (87.1%). The presence of ≥two non-pulmonary organ manifestations increased the likelihood of an underlying systemic disease by three to five-fold. Conclusions: An etiological diagnostic strategy effectively classifies chILD and guides genetic testing. Exome or genome sequencing should be considered if ≥two non-pulmonary organs are involved or if the initial diagnosis becomes uncertain due to an unusual disease course or signs of a second underlying condition.