Pulmonary function tests sometimes indicate a progressive deterioration and at other times a 'stepwise' worsening which may be followed by improvement. Interpretation depends on the extent of random or diurnal variations in function. Routing pulmonary function tests (VC, FEV1, FRC, and airway resistance (Raw)) were repeatedly measured in normal subjects, patients with stable irreversible airways obstruction, and patients with stable restrictive disease. In all groups there was a significant (P less than 0.001) diurnal variation in Raw, with high values in the morning, low values at noon, and rising values in the evening. The midday Raw values were about 80% of the highest daily values. The considerable random and diurnal variability seen in all tests is reflected in the range of high and low values (% of mean individual response) in individuals. The largest variation in an individual between measurements taken at two different times was 81% in Raw (range: 40% above to 41% below the mean). There was less variation in FEV1 (29%), FRC (62%), and VC (30%). Thus the finding of a stepwise change in function could reflect its natrual variability. When repeated studies are done to assess progress or the effects of therapy on disease, there are many factors, including the time of day at which the tests are performed, which should be standardized as far as possible.
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