Evaluation of occupational and leisure timed exposure assessment in a population-based case control study on leukaemia

Objectives: To increase the credibility of retrospective exposure assessmen ts, documentation of the procedures and presentation of measures on quality control is recommended. The aim of this study was to present and evaluate the procedures used in a case-control study on leukaemia. Methods: A series of 1,087 cases and matched controls were interviewed. Exposure assessments were performed for 13 occupational agents and ten leisure-time activities and the confidence of the assessments was coded. The exposure assessments w ere performed blind by three occupational hygienists. Ten percent of the in terviews were reassessed by two of the three occupational hygienists. Resul ts: The leisure-time activities contributed to a large extent to the overal l prevalence of exposure. For organic solvents approximately 25% of the con trols classified as exposed would be misclassified if leisure-time exposure were not considered. The proportions of subject assessments with low confi dence were higher for next-of-kin than for in-person interviews. A negative correlation was seen between the proportions of assessments with low confi dence and the reliability. A significant difference was seen in the inter-r ater comparison between cases and controls when the reliability was calcula ted for each assessed period; no difference was seen for the subject assess ments used for relative risk estimation. Conclusion: When low-dose exposure in epidemiological studies are being assessed there is an obvious risk of misclassification if leisure-time activities are not included. Furthermore, the reliability of the assessments may suffer if next-of-kin interviews ar e used to a large extent. For cancers with poor prognoses, prospective stud ies are preferable to minimise possible information bias.