This study aims to establish normative values for respiratory oscillometry in the Polish pediatric population. It is designed as a multicentre, population-based, prospective cohort study including children and adolescents aged 3-18 years. The results will provide reference standards for respiratory oscillometry that may support the diagnosis and monitoring of respiratory diseases in clinical practice and research.

OSCILLONiCA-PL-Kids is a multicentre, observational, cross-sectional study designed to establish normative reference values for respiratory oscillometry (IOS) in healthy Polish children and adolescents aged 3-18 years. The study is coordinated by the Polish Society of Pediatric Pulmonology and conducted in 12 pediatric centers across Poland between 2023 and 2025.

Standardized Jaeger oscillometry systems (ERS-validated) are used across all sites, following harmonized operating procedures. All investigators and technicians undergo centralized training and certification, with periodic audits and refresher sessions to ensure procedural consistency and data quality. Measurements are performed during tidal breathing with cheek support and a nose clip, recording at least three technically acceptable trials per participant. Parameters include resistance (Rrs) and reactance (Xrs) at 5, 10, and 20 Hz, resonance frequency (Fres), and area of reactance (AX).

Anthropometric data (height, weight, age, sex) are collected simultaneously to allow modeling of predicted values. Data integrity is verified using automated quality-control algorithms. Reference equations for oscillometric parameters will be derived using generalized additive models for location, scale, and shape (GAMLSS, LMS method). Model performance will be evaluated using the Schwarz Bayesian Criterion, with sensitivity analyses assessing robustness to borderline cases and environmental exposures.

Raw measurement data, as well as derived parameters (means, standard deviations, percentiles, and z-scores), will be used to generate normative reference values and percentile charts stratified by age, height, and sex.

Quality assurance: Data collection and management follow standardized operating procedures (SOPs) approved by the coordinating center. Each site conducts internal data validation prior to upload, and periodic cross-site audits are performed to ensure compliance and consistency. Automatic range and logic checks are implemented in the database to flag out-of-range or inconsistent entries.

Missing data plan: Incomplete or invalid recordings will be excluded from normative modeling. Sensitivity analyses will evaluate the impact of missing data on reference equations. Imputation will not be used for normative dataset derivation.

The study adheres to ERS Technical Standards for Oscillometry (2020) and complies with Good Clinical Practice for observational research. The resulting reference equations and percentile charts will provide validated normative standards for pediatric oscillometry in Poland and support future international harmonization efforts.

Inclusion Criteria:1. Age between 3 and 18 years. 2. Caucasian race (Born and residing in Poland). 3. Absence of chronic respiratory disease or acute respiratory symptoms at the time of testing.

4. Written informed consent from a parent/legal guardian (for participants <16 years) or from the participant if ≥16 years.

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Exclusion Criteria:1. History of chronic lung disease (e.g., asthma, cystic fibrosis, bronchopulmonary dysplasia, interstitial lung disease).

2. Acute respiratory infection within 4 weeks before testing. 3. Severe chronic comorbidities (e.g., congenital heart disease, neuromuscular disorders).

4. Chest deformation 5. Inability to perform oscillometry according to ERS standards.

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Healthy Polish children and adolescents aged 3 to 18 years. Participants are recruited from schools, kindergartens, and pediatric outpatient clinics across 12 centers in Poland. Only individuals without chronic respiratory diseases or recent respiratory infections are eligible.

The study team will consider sharing de-identified individual participant data after publication of the primary results, depending on institutional policies and ethical approval.