The within subject variability of forced vital capacity (FVC), forced expiratory volumes in one second (FEV1) and half a second (FEV0.5), peak expiratory flow (PEF), and flow rates at 25-75%, 75-85%, 25%, 50%, and 75% of expired FVC were assessed among 7 year old children from the general population. Within occasion variability in 232 children was lowest for FVC (coefficient of variation (CV) 5%) and FEV1 (CV 4%), and greatest for end expiratory flow rates. The precision of measurement for FEV1 supports its use for bronchial provocation tests, particularly those using a graded challenge. In this context the value of PEF (CV 7%) and mid expiratory flow rates (CV 11%) is limited by their poorer repeatability. Between occasion variability was assessed in 171 children tested at an interval of one to four weeks. The difference between the variances between occasions and within occasions was attributed to biological variation; this accounted for a substantial component of the between occasion variance in all indices, particularly FEV1 (73%) and PEF (66%). Together, within subject variability, sex, and height accounted for about half of the measured variance between subjects for all indices except FVC (68%). These results have implications for epidemiological studies.
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