Derivation and validation of spirometry reference equations for Jordanian preschool children using GAMLSS modelling

To formulate spirometry reference equations that can be implemented on preschool-aged children in Jordan, while accounting for their age, sex and height.

This two-phase Jordanian study derived sex-specific spirometry reference equations for healthy children aged 3-6 years and evaluated their temporal performance in an independent previously published Jordanian preschool cohort. In the 2025 model-development cohort, generalized additive models for location, scale and shape (GAMLSS) were fitted separately for boys and girls for forced expiratory volume in one second (FEV₁), forced vital capacity (FVC), and FEV1/FVC, comparing Box-Cox Cole and Green distribution (BCCG), Box-Cox Power Exponential distributions (BCPE), and Normal families and selecting models using GAIC with diagnostic assessment by quantile residuals and worm plots. In the validation cohort described by Al-Qerem and Jarab, calibration and external performance were evaluated and compared against Global lung initiative equations (GLI-2012 Caucasian and GLI-2022 race-neutral equations).

The study included 1676 children: derivation cohort, n = 911, 461 boys; validation cohort, n = 765, 400 boys. BCPE models provided the best fit for FEV₁ and FVC in both sexes, whereas FEV₁/FVC was adequately captured by simpler models (boys: BCCG; girls: Normal). The validation phase indicated minimal systematic bias (intercepts near zero; slopes close to one), although FEV₁/FVC results were less precise. Jordanian and GLI-2012 yielded mean volume z-scores near zero, whereas GLI-2022 shifted volume estimates upward and reduced lower limit of normal yields, particularly for girls' FEV₁.

These first Jordanian preschool spirometry equations demonstrate good performance and support population-specific interpretation in clinical practice.