Aims to compare Specific inhalation challeng (SIC) with serial measurements of PEF in the diagnosis of occupational asthma Methods; All workers having SIC with occupational agents over a 3 year period were included. Their serial PEF records made during exposure to the suspected agents where analysed using Oasys software. Positive records were those with any of the following; Oasys score >2.5; ABC score ≥15litres/min/hr or timepoint ≥1 non-waking reading Results; 211 challenges were done in 51 workers. 45/51 kept serial PEF records suitable for Oasys analysis. SIC and Oasys analysis were concordant in 17/45 (38%), particularly those exposed to isocyanates or metal-working fluids. SIC was positive in 5 workers with equivocal Oasys analysis in line with its known sensitivity of c70–80%. 12 workers had negative or non-asthmatic SIC responses with positive Oasys analysis. Further investigation showed that occupational asthma was the most likely diagnosis. Negative SIC responses were due failure to identify the correct causative agent or problems with reproducing the work exposures. This was a particular problem with cleaning agents where a protein source may be needed to convert chlorine-releasing agents to chloramines (as shown in swimming pool asthma). Nine workers had equivocal challenges and clearly positive Oasys analysis, helping to clarify the diagnosis in this group, again non-standard agents were common in this group.

Conclusion SIC and serial PEF analysis are complementary methods for validating a diagnosis of occupational asthma. SIC has particular problems when methods of exposure for newer agents have not been fully developed, Oasys analysis lacks sensitivity when current specificity is fixed at >90%.