Why is evidence on job strain and coronary heart disease mixed? An illustration of measurement challenges in the Whitehall II study.

OBJECTIVE: Evidence regarding the status of job strain as a risk factor for coronary heart disease (CHD) is mixed, including both results supporting the risk status and null findings. However, previous studies have typically assessed job strain at one point in time only. We examined whether the failure of such measurement to reflect long-term job strain could contribute to false null findings.
METHODS: Job strain and its components, as stress indicators, were assessed twice (3-year time lag) for 5043 men and 2210 women who were free of apparent CHD at baseline. Incident CHD after the stress measurement comprised CHD death, a first nonfatal myocardial infarction, or definite angina (mean follow-up, 10.4 years). The data analysis was based on Cox proportional-hazard models adjusted for age, sex, and employment grade and corrected using regression dilution ratios calculated from short-term repeat data in a random subsample.
RESULTS: In the total cohort, incidence of new CHD was higher for higher levels of job strain and demands. For these stress indicators, the corrected excess CHD risk was 30% and 29% higher than the corresponding uncorrected estimates, whereas the corresponding increase for job control was only 13%. Effects of job strain and work demands, but not job control, were stronger for a subgroup, with consistent exposure measurements over time than for the total cohort.
CONCLUSION: This evidence suggests that use of single-time exposure measures may underestimate the status of long-term job strain as a CHD risk factor.