Abstract
Determine the contribution of secular trends and sample size to lung function reference equations, and establish the number of local subjects required to validate published reference values.
Thirty spirometry datasets, collected between 1978–2009, provided data on healthy whites: 19,291 males and 23,741 females, 2.5–95 years. The best fit for FEV1, FVC and FEV1/FVC as functions of age, height and sex were derived from the entire dataset using GAMLSS. Mean z-scores were calculated for individual datasets to determine inter-centre differences. This was repeated by subdividing one large dataset (3,683 males, 4,759 females) into 36 smaller subsets (comprising 18–227 individuals) to preclude differences due to population/technique.
No secular trends were observed, and differences between datasets comprising >1000 subjects were small (maximum difference in FEV1 and FVC from overall mean: +0.30 to −0.22 z-scores). Subdividing one large dataset into smaller subsets reproduced the above sample size related differences and revealed that at least 150 males and 150 females would be necessary to validate reference values to avoid spurious differences due to sampling error.
Use of local controls to validate reference equations will rarely be practical due to numbers required. Reference equations derived from large or collated datasets are recommended.
- ERS
