[A case control study for the evaluation of the cytological detection of cancer of the cervix].

A case control study was carried out in Geneva to verify the efficacy of early detection of cancers of the cervix uteri by cytology and to estimate the degree of protection. The study included the 186 incident cases of invasive cancer occurring in the time period 1970-76 and also fulfilling the condition of having lived at least 10 years in Geneva. Each case is matched to one control according to age, nationality (Swiss/foreigners), civil state (unmarried/other). The control had also to be resident for ten years. The analyse was based on the number of negative smears during the observation period; this period commenced 10 years before the date of incidence but not earlier than 1.1.64. The observation period ended when there was a positive smear or when there was an interval of less than 6 months between two negative smears. Each observation period is the same for both case and control. The protection conferred is significant and is estimated to be 3.2 (1.8-5.9) for women having had at least one negative smear. On the other hand, the degree of protection tends to diminish in direct proportion to the number of negative smears. This paradoxical result may be attributed to a selection effect of symptomatic cases among women undergoing repeated cytological examinations.