Objectives: This study examines whether health-related selection out of shift work is likely to bias the association between shift work and cardiovascular disease (CVD).
Methods: Survey data on doctor-diagnosed CVD (myocardial infarction, angina, or hypertension) and risk factors (high blood pressure, high cholesterol concentration, obesity, and diabetes) were collected in 2000-2002 for a cohort of 7037 female nurses (5038 shift workers, 1999 day workers) in 21 Finnish hospitals. The follow-up in 2004 determined those who had left their workplace or changed from shift work to day work.
Results: Among the shift workers, the age-adjusted odds for leaving the organization was 1.83 (95% CI 1.01-3.32) times higher for those with prevalent diabetes and 2.21 (95% CI 1.12-4.39) times higher for those with three to four risk factors than for their counterparts with no diabetes or risk factors. The associations between CVD risk factors and leaving the organization were similar for the day workers. The prevalent CVD and risk factors did not predict a change to day work among the shift workers who remained in the organization during the entire follow-up period.
Conclusions: Employees with several risk factors are more likely to leave an organization regardless of the type of work schedule. Health-related selection out of shift work is an unlikely source of major bias in research on shift work and CVD.