The reproducibility of some respiratory function tests and a simple self-paced walking test were assessed in 13 patients with stable chronic obstructive airways disease by making observations on six occasions at two to three weekly intervals. The coefficients of variation of the various tests were as follows: forced expiratory volume in one second, +/- 14.8%; forced vital capacity, +/- 11.1%; transfer factor (single breath carbon monoxide), +/- 15%; total lung capacity, +/- 8.8% by helium dilution and +/- 8.3% by body plethysmography; increase in heart rate and minute ventilation for an increase in oxygen uptake of 0.5 1 min-1, +/- 19.4% and +/- 16.3%; distance covered in a 12-minute walking test, +/- 8.2%. The 12-minute distance increased significantly over the first three studies. The considerable variations in the result of these tests in subjects with apparently stable clinical states and without changes in treatment should be considered when assessing the results of changes in treatment.
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