Diffusing capacity and forced vital capacity in 5,003 asbestos-exposed workers: Relationships to interstitial fibrosis (ILO profusion score) and pleural thickening

Background: Asbestosis is an interstitial lung disease whose radiographic severity has long been graded by the International Labour Office (ILO) profusion score. Its effect on pulmonary function is further impacted by asbestos related pleural thickening. Objectives: This report aims to describe the relationships between radiographic grading of interstitial and pleural fibrosis and a key test of pulmonary function, the diffusing capacity, which measures gas exchange and has rarely been assessed in large groups, and to confirm the relationship to an independent test of pulmonary function, the vital capacity, which measures a mechanical property of the lungs. Methods: The data were derived from a survey during the period 1997-2004 of 5,003 workers (all white males) exposed to asbestos in various trades. Tests included chest radiographs read by a certified expert ("B") reader, forced vital capacity (FVC), and carbon monoxide diffusing capacity (DLco). Cigarette smoking was adjusted for in the predictive equation for DL_CO. Workers were evaluated at a mobile facility at work sites in four southern states. Results: Both diffusing capacity and vital capacity were negatively correlated with profusion score over the full spectrum of radiographic severity. ILO profusion scores 0/1 (conventionally classified as normal) and 1/0 (conventionally classified as abnormal) were associated with similar diffusing capacity and vital capacity values. The highest profusion scores were associated with a greater proportionate decrease in diffusing capacity than in FVC. Both tests showed an effect of pleural fibrosis. Conclusions: Both radiographic severity graded by the profusion score and pleural thickening are correlated with two independent measures of pulmonary function. FVC (which had been reported in smaller work forces) and DL_CO (which has not been reported). Both measures show a decrease from normal to intermediate (0/1, 1/0) scores and a further decrease with greater scores, demonstrating the consistency of radiographic and functional assessments.