Natural variability of lung function in young healthy school children

Abstract

Knowledge about long-term variability of lung function in healthy children is essential when monitoring and treating those with respiratory disease over time. The aim of this study was to define the natural variability in spirometry in young children after an interval of 12 months.The Size and Lung function In Children study was a prospective study designed to assess spirometry and body size, shape and composition in a multi-ethnic population of London school children. 14 schools with a wide range of socioeconomic circumstances were recruited. Spirometric and anthropometric assessments and parental questionnaires pertaining to respiratory symptoms, previous medical history, pubertal status and socioeconomic circumstances were completed at baseline and ∼1 year later.Technically acceptable spirometry data on two occasions ∼1 year apart (range 9-16 months) were available in 758 children (39% boys, mean±sd age 8.1±1.6 years), 593 of whom were classified as "healthy". Mean±sd within-subject between-test variability was 0.05±0.6 z-scores, with 95% of all the children achieving a between-test variability within ±1.2 z-scores (equating to ∼13% predicted).Natural variations of up to 1.2 z-scores occur in healthy children over ∼1 year. These must be considered when interpreting results from annual reviews in those with lung disease who are otherwise stable, if unnecessary further investigations or changes in treatment are to be avoided.

Figure 1

Flow chart for subjects included in the study.

Figure 2. Change in FEVI Z-score (visit 2-visit 1) compared with the FEVI z-score on the initial visit

Dashed line indicates lower limit of normal (<−1.64z-scores).

Figure 3. Between-test (TI-T2) difference in FEV₁ z-scores versus change in height z-score (T2-T1), according to pubertal status for the reference population

Dashed lines indicate the 95% Limits of Agreement (±1▪2 z-scores). The lack of association between differences in FEV₁ z-scores and height z-scores, and negligible effects of pubertal status suggest that these factors have a minimal role in explaining the more extreme changes seen in a few children.