Background: The diagnosis of occupational asthma requires objective confirmation. Analysis of serial measurements of Peak Expiratory Flow (PEF) is usually the most convenient first step in the diagnostic process. We describe a new method of analysis originally developed to detect late asthmatic reactions following specific inhalation testing and have applied it to serial PEF measurements made over many days in the workplace, to supplement existing methods of PEF analysis.
Methods: 236 records from workers with independently diagnosed occupational asthma and 320 from asthmatic controls were available. The pooled standard deviation for rest day measurements was obtained from an analysis of variance by time. Work day PEF measurements were meaned into matching 2-hourly time segments. Timepoints with mean workday PEF statistically lower (at the Bonferroni adjusted 5% level) than the rest days were counted, after adjusting for the number of contributing measurements.
Results: A minimum of 4 timepoint comparisons were needed. Records with ≥2 timepoints significantly lower on workdays had a sensitivity of 67% and a specificity of 99% for the diagnosis of occupational asthma against independent diagnoses. Reducing the requirements to ≥1 non-waking timepoint difference increased sensitivity to 77% and reduced specificity to 93%. The analysis was only applicable to 43% of available records, mainly due to differences in waking times on work and rest days.
Conclusion: Timepoint analysis complements other validated methods of PEF analysis for the diagnosis of occupational asthma. It requires shorter records than required for the Oasys score and can identify smaller changes than other methods, but is dependent on low restday PEF variance.