Introduction: Allergic sensitisation and lower respiratory tract infection (LRTI) during the first years of life are associated with adverse in lung function in school age children but their effect on preschool lung function remains poorly understood.

Aim: To estimate the effect of allergic sensitisation and LRTI in the first year of life on gas mixing properties of the lung at age 4 years.

Methods: Data were from the Barwon Infant Study, a birth cohort study of 1074 infants from the Barwon region of Victoria, Australia. Participants completed skin prick testing for allergic sensitisation at age 1 year, questionnaires on respiratory illness between birth and age 2 years and nitrogen multiple breath washout (N₂MBW) assessment at 4 years of age. Outcomes were lung clearance index (LCI) and functional residual capacity (FRC). Differences in mean for LCI and FRC between respiratory illness and sensitisation groups were estimated using linear regression, with adjustment for confounding.

Results: N₂MBW data were available in 284 children at age 4 years. Allergic sensitisation at 1 year of age was associated with higher LCI at 4 years of age (Mean difference [MD], 95% confidence interval [CI]: 0.26, 0.00, 0.53). This effect was greater among children who experienced LRTI (MD, 95% CI: 0.32, 0.08, 0.57). LCI was similar between those with and without LRTI (MD, 95% CI: 0.05, -0.07 to 0.17).

Conclusion: At 4 years of age, allergic sensitisation in infancy was associated with greater ventilation inhomogeneity, and this effect was more pronounced in children who also had LRTI during infancy. There was no evidence LRTI alone impacted subsequent lung function.