Occupational Asthma Reference
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Moore VC, Walters GI, Robertson AS, Burge PS,
Identification of late asthmatic reactions following specific inhalation challenge,
Occup Environ Med,
2020;77:728-731,doi.org/10.1136/oemed-2020-106436
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Keywords: occupational asthma, challenge, method, key, late reaction, uk
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Abstract
Specific inhalation challenge (SIC) is the reference standard for the diagnosis of occupational asthma. Current guidelines for identifying late asthmatic reactions are not evidence based.
Objectives
To identify the fall in forced expiratory volume in 1 s (FEV1) required following SIC to exceed the 95%?CI for control days, factors which influence this and to show how this can be applied in routine practice using a statistical method based on the pooled SD for FEV1 from three control days.
Methods
Fifty consecutive workers being investigated for occupational asthma were asked to self-record FEV1 hourly for 2 days before admission for SIC. These 2?days were added to the in-hospital control day to calculate the pooled SD and 95%?CI.
Results
45/50 kept adequate measurements. The pooled 95%?CI was 385?mL (SD 126), or 14.2% (SD 6.2) of the baseline FEV1, but was unrelated to the baseline FEV1 (r=0.06, p=0.68), or gender, atopy, smoking, non-specific reactivity or treatment before or during SIC. Thirteen workers had a late asthmatic reaction with =2 consecutive FEV1 measurements below the 95%?CI for pooled control days, 4/13 had <15%?and 9/13 >15%?late fall from baseline. The four workers with =2 values below the 95%?CI all had independent evidence of occupational asthma.
Conclusion
The pooled SD method for defining late asthmatic reactions has scientific validity, accounts for interpatient spirometric variability and diurnal variation and can identify clinically relevant late asthmatic reactions from smaller exposures. For baseline FEV1 <2.5?L, a 15% fall is within the 95%?CI
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Comments
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Specific inhalation tests are more difficult to perform in workers who have isolated late asthmatic reactions, as more caution is needed increasing exposures. Defining a late asthmatic reaction outside the 95% CI for control days allows identification of late reactions with lower percentage change in FEV1 in those with initial values >2.5 litres, however 3 control days are needed. This paper reports the results when 2 of the 3 control days are done unsupervised at home, with the third day following control exposure in the laboratory. The pooled SD method for defining late asthmatic reactions has scientific validity, accounts for interpatient spirometric variability and diurnal variation.
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