The intrasubject variability of repeat pulmonary function testing was examined in 20 healthy children aged 10 to 16 years. The children were tested a maximum of 11 times over a period of two months. The tests examined were spirometry, maximum expiratory flow-volume curves, body plethysmograph determination of lung volumes, and single breath nitrogen washout. The time of day or the length of the re-test interval, up to a period of two months, did not significantly affect the variability. Standard deviation was used when comparing the variability of measurements with the same units and coefficient of variation was used when comparing measurements of different units. The vital capacity measured by spirometry was the least variable measurement. Functional residual capacity, residual volume and total lung capacity were equally reproducible but as a group were more variable than vital capacity. There was no significant difference between the variability of the following measurements: forced expired flow from 25 to 75% of the vital capacity, flows at 70%, 50% and 40% total lung capacity, and flows at 50% vital capacity. Flows at 25% vital capacity were significantly less variable than other measurements of flow. The variability of forced expired volume in one second was examined and the use of this measurement in determining significant bronchial hyper-reactivity was assessed.
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