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Paediatric respiratory disease

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1S. Stanojevic, 1T. J. Cole, 1A. Wade, 1C. Bastardo, 2A. Prasad, 3P. H. Quanjer, 1J. Stocks. 1UCL Institute of Child Health, London, UK, 2Great Ormond Street Hospital, London, UK, 3Sophia Children’s Hospital, Rotterdam, The Netherlands

Background: Serial spirometry measurement can facilitate the identification of individuals with rapid lung function decline. In clinical practice, however, repeated measurements are commonly interpreted using cross-sectional reference data, despite well recognised limitations (Van Pelt et al. AJRCCM 1994).

Aim: To assess adjustments for the correlated nature of repeated measurements in order to interpret changes over time using cross-sectional reference data.

Methods: Normal lung growth can be viewed as roughly a constant Z-score (adjusted for height, age and sex) over time, with the uncertainty around the change normally distributed around a mean change of 0 Z-scores. The standard deviation of the change depends on the correlation (r) between the two measurements (Cole, Human growth in context, 1998). The adjusted Z-score for change (Zc) adjusts the observed Z-score change for the baseline Z-score and the interval between measurements. Spirometry data from 938 healthy, white subjects (aged 5–18 years) measured twice over time intervals 0.1 and 5 years, were collated to estimate the occasion-to-occasion correlation coefficient (r) for FEV1 Z-score (Stanojevic et al. AJRCCM 2008). Measurements less than one year apart had an r value of 0.90, such that normal within-subject variability in health after one year is 0 ± 0.88 Z-scores. Changes in spirometry were also assessed after a one-year interval in 98 children with cystic fibrosis (aged 4–12 years) to determine the clinical implications of using the Zc.

Results: Based on a cross-sectional cut-off of <−1.64 Z-scores, 32/98 patients had an abnormal result at baseline, of whom 21 remained abnormal after one …

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