Objectives To identify the changes in serial 2-hourly forced expiratory volume in 1 s (FEV₁) measurements required to identify occupational asthma (OA) using the Oasys Area Between Curves (ABC) score.

Methods The ABC score from 2-hourly measurements of FEV₁ was compared between workers with confirmed OA and asthmatics without occupational exposure to identify the optimum separation using receiver operator characteristic (ROC) analysis. Separate analyses were made for plots using clock time and time from waking to allow for use in shift workers. Minimum record criteria were ≥6 readings per day, >4 day shifts and >4 rest days (or >9 days for controls).

Results A retrospective analysis identified 22 workers with OA and 30 control asthmatics whose records reached the quality standards. Median FEV₁ diurnal variation was 20.3% (IQR 16.1–32.6) for OA and 19.5% (IQR 14.5–26.1) for asthmatic controls. ROC curve analysis identified that a difference of 0.056 L/hour gave a ROC score of 0.821 for clock time and 0.768 for time from waking with a sensitivity of 73% and a specificity of 93% for the diagnosis of OA.

Conclusions The diagnosis of OA requires objective confirmation. Unsupervised serial FEV₁ measurements are more difficult to obtain reliably than measurements of peak expiratory flow, which are likely to remain the standard for general use. A FEV₁ ABC score >0.056 L/hour provides a valid cut-off for those who wish to use FEV₁ rather than peak expiratory flow.