Case-control studies of screening.

The application of case-control studies to the evaluation of screening data presents a number of potential problems. In particular, the results are liable to be affected by selection bias. Published case-control studies of breast cancer screening are reviewed in the light of the results of a recent UK study of somewhat different design which attempts to estimate the effect of selection bias. Case-control studies of cervical screening involve additional difficulties, such as the handling of cases of invasive disease diagnosed by screening, and the distinction between diagnostic and screening smears. The extent to which published studies have addressed these is discussed. It is concluded that case-control studies of screening should be interpreted with caution, particularly in situations where comparable data from controlled trials are not available.