Identification of late asthmatic reactions following specific inhalation challenge

Abstract

Background: Specific inhalation tests (SIC) aim to identify the specific cause of asthma with sufficient certainty to exclude measurement inaccuracy and spontaneous diurnal changes, while minimising deterioration in the workers asthma. We aimed to define late asthmatic reactions using the pooled SD method of Stenton in consecutive workers undergoing SIC. Methods: Two unsupervised control days at home were added to an in-hospital control day to provide the three control days required to calculate the pooled FEV1 SD. A late asthmatic reaction required two consecutive measurements below the 95% CI for the control days. The absolute change in ml and the change expressed as % baseline were calculated. Results: Sufficient FEV1 data were provided by 45/50 workers. The mean 95% CI for control day FEV1 was 385ml (SD 126), independent of FEV1, treatment and personal characteristics. Four workers had late asthmatic reactions identified only by the pooled SD method, all confirmed by independent methods. The 95% CI for the FEV1 fall expressed as a percent baseline value was 14.2% (SD 6.2%), correlated inversely with the baseline FEV1 (r=0.55, p Conclusions: The pooled standard deviation method for defining late asthmatic reactions has scientific validity, accounts for inter-patient spirometric variability and diurnal variation, and can identify clinically relevant late asthmatic reactions from smaller exposures. It warrants inclusion in guidelines.