Case and control recruitment, and validation of cases for the MICA case-control study in England, Scotland and Wales

Objectives - To describe the processes involved in the selection of cases a nd controls, and the validation of case diagnosis, in a large case-control study on the association between myocardial infarction (MI) and oral contra ceptives. Design - Community case-control study in England, Scotland and Wales. Subjects - Cases were women aged 16-44 with a diagnosis of MI. Controls wer e age and general practice matched women without a diagnosis of MI. Results - The proportion of interviewed cases per number eligible was 19% ( 95% CI 12-26%) lower in England and Wales than in Scotland. The kappa score for agreement between three cardiologists on diagnostic categories of the potential cases was 0.52 overall ('moderate' agreement). For the 35 cases r esubmitted for categorization, the kappa scores for agreement between the f irst and second classification for the two national committee cardiologists were 0.32 and 0.23 ('fair' agreement). Age matching of controls was better in Scotland than England and Wales, the proportion of the first choice con trols being 14% (95% CI 8-19%) higher in Scotland. The mean time to complet e a cluster of intel views (one case and four controls) was 36 days in Scot land and 63 days in England and Wales. Conclusions - Very close age-matching was achieved overall, although the pr ocess was more efficient in Scotland than in England and Wales. Some sugges tions for improvement in the case validation process are made. Copyright (C ) 1999 John Wiley & Sons, Ltd.