1. We performed a chronobiological study of the relationship between peak expiratory flow rate (PEFR) and magnitude of dyspnoea in 35 symptomatic asthmatic patients to determine how accurately asthmatic subjects assess spontaneous airway obstruction and whether this accuracy varies throughout the 24 h period and depends on characteristics of the subjects or asthma.

2. At 07.00,11.00, 15.00, 19.00 and 23.00 hours on 8 consecutive days in their ordinary environment and under their usual drug regimen, the subjects first rated their dyspnoea with a visual analogue scale and immediately after recorded their PEFR.

3. The linear regression coefficients between dyspnoea score and PEFR for the 35 subjects were continuously distributed between −0.93 and +0.21, with most r values ranging from −0.9 to −0.5. The group median r values calculated at each time point showed that strongest correlation between dyspnoea score and PEFR occurred at 7 h, which coincided with the lowest PEFR values.

4. We chose r = −0.7 (r2 = 0.5) as a limit to distinguish good perceivers (−1 < r ≤ −0.7) from bad perceivers of airway obstruction. These two populations differed only by a higher variability of both PEFR and dyspnoea score in good than in bad perceivers, but not for severity, duration or treatment of asthma, absolute value of dyspnoea score, age or sex.

5. Our study quantified an important intra- and between-subject variability in the accuracy of perception of spontaneous airway obstruction in asthmatic subjects which seems to be unrelated to most characteristics of asthma.

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