Factors affecting the long-term variability of bronchial responsiveness in an adult general practice population.

There have been few longitudinal studies of bronchial responsiveness. We wanted to assess the long-term variability and associations of bronchial responsiveness in the general population. Spirometry, bronchial provocation tests, skin-prick tests for allergy, and respiratory symptom questionnaires were repeated every 4 months, for 2 years (August 1987-August 1989), in 122 volunteers recruited from a cross-sectional survey of population. Provocation dose producing a 20% fall in forced expiratory volume in one second (PD20FEV1) and dose-response slope (SL), which gives values for methacholine responsiveness, were measured in all subjects. SL correlated well with PD20FEV1 but repeatability was impaired in those subjects with unmeasurably high PD20FEV1. The 95% range for repeatability of PD20FEV1 was +/- 3.11 doubling doses and +/- 4.52 doubling slopes for SL. Bronchial responsiveness increased in those with self-reported colds and reduced FEV1 in winter 1987-1988, and in males in winter 1988-1989. Bronchial responsiveness increased during the summer (June-August) of both years, significantly in year 1. We conclude that bronchial responsiveness showed minor seasonal variability and that colds were the strongest predictors of increased bronchial responsiveness over the 2 yr period.