Short and long term reproducibility of responsiveness to inhaled histamine: Rt compared to FEV1 as measurement of response to challenge.

Repeatability (123 min) and reproducibility (65 days) of response to inhaled histamine were tested in a selected group of stable asthmatics and non-asthmatics by means of FEV1 and breathing resistance (Rt) determined by the interruptor method. Variation in prechallenge FEV1 and Rt was kept within a +/- 10% and +/- 40% range, respectively. We found the PC20-FEV1 highly repeatable after 123 min. The 95% confidence interval for a single determination was the measured value +/- 1.72 two-fold concentration differences. Reproducibility was significantly lower (p less than 0.05). Both repeatability and reproducibility of PC30-Rt were significantly lower than that of PC20-FEV1. Interruptor method determined PC30-Rt was found repeatable within 4 two-fold concentration differences which is not acceptable for clinical and research purposes. The poor repeatability of PC30-Rt could be due to changes in functional residual capacity during challenge.