Prediction of dropout from respiratory symptoms and airflow limitation in a longitudinal respiratory study.

OBJECTIVES

This study investigated the possibility that employees reporting respiratory symptoms were more likely than asymptomatic workers to dropout of a respiratory study carried out in Norwegian smelters.

METHODS

The study included 3924 employees in 24 Norwegian smelters. They were examined annually using a respiratory questionnaire and spirometry. The employees who did not meet for the follow-up within 18 months prior to the end of the study were considered dropouts. The data were analyzed using Cox regression for time-dependent covariates.

RESULTS

The total and the median follow-up times were 16 997 and 4.9 years, respectively. The overall dropout rate was 44.5 [95% confidence interval (95% CI) 41.5-47.8 per 1000 person-years]. The hazard ratio (HR) for dropout was 1.38 (95% CI 1.15-1.66) for the workers reporting any respiratory symptom compared with the asymptomatic workers. The effect was the strongest among the employees who reported dyspnea, and it was stronger regarding symptoms at the last visit than for the baseline symptoms. Similarly, the hazard ratio for dropout for those with an airflow limitation [forced expiratory volume/forced vital capacity below the 5th percentile of the predicted value] was 1.31 (95% CI 1.01-1.69) when they were compared with employees without any airflow limitation.

CONCLUSIONS

Respiratory symptoms and airflow limitation are important predictors of dropout from a longitudinal respiratory study.