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

OBJECTIVE

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

METHODS

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).

RESULTS

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.

CONCLUSION

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