Objective Byssinosis remains a significant problem among textile workers in low/middle-income countries. Here we share our experience of using different prediction equations for assessing ‘chronic’ byssinosis according to the standard WHO classification using measurements of forced expiratory volume in 1 s (FEV₁).

Methods We enrolled 1910 workers in a randomised controlled trial of an intervention to improve the health of textile workers in Pakistan. We included in analyses the 1724 (90%) men who performed pre-bronchodilator spirometry tests of acceptable quality. We compared four different equations for deriving lung function percentage predicted values among those with symptoms-based byssinosis: the third US National Health and Nutrition Examination Survey (NHANES-III, with ‘North Indian and Pakistani’ conversion factor); the Global Lung Function Initiative (GLI, ‘other or mixed ethnicities’); a recent equation derived from survey of a western Indian population; and one based on an older and smaller survey of Karachi residents.

Results 58 men (3.4%) had symptoms-based byssinosis according to WHO criteria. Of these, the proportions with a reduced FEV₁ (<80% predicted) identified using NHANES and GLI; Indian and Pakistani reference equations were 40%, 41%, 14% and 12%, respectively. Much of this variation was eliminated when we substituted FEV₁/forced vital capacity (FVC) ratio (<lower limit of normality) as a measure of airway obstruction.

Conclusion Accurate measures of occupational disease frequency and distribution require approaches that are both standardised and meaningful. We should reconsider the WHO definition of ‘chronic’ byssinosis based on changes in FEV₁, and instead use the FEV₁/FVC.