Optimal frequency of screening for cervical cancer: a Toronto case-control study

A retrospective (case-control) study was conducted in Toronto, Canada, between 1973 and 1976 to evaluate the effectiveness of the Papanicolaou smear as a screening procedure for invasive carcinoma of the cervix. In the current study, data from cases of squamous cell cancer of the cervix and controls without hysterectomy were analysed to assess the optimal frequency of cervical screening. The time between the last normal smear and the date of diagnosis was divided into 12-month intervals. Relative protections were then calculated by comparing women who had had one or more normal smears with those who had never been screened. The results show a significant protective effect of screening in women who have had two or more normal smears, when the last was taken within two years of diagnosis. This trend, although not statistically significant, continued even when the last was taken four years before diagnosis, suggesting that screening intervals should not be greater than four years.