Outcomes of screening to prevent cancer: analysis of cumulative incidence of cervical abnormality and modelling of cases and deaths prevented.

OBJECTIVE: To determine the frequency of different outcomes in women participating in cervical screening. DESIGN: Analysis of screening records from 348 419 women, and modelling of cases of cervical cancer and deaths with and without screening. SETTING: Cervical screening programme in Bristol. RESULTS: For every 10 000 women screened from 1976 to 1996, 1564 had abnormal cytology, 818 were investigated, and 543 had abnormal histology. One hundred and seventy six had persistent abnormality for two years or more. In the absence of screening 80 women would be expected to develop cancer of the cervix by 2011, of whom 25 would die. With screening 10 of these deaths would be avoided. Comparison of cumulative abnormality rates with numbers expected to develop cancer in the absence of screening suggests that at least 80% of high grade dyskaryosis and of high grade dysplasia would not progress to cancer. The lifetime risk of having abnormal cytology detected could be as high as 40% for women born since 1960. CONCLUSIONS: Screening is labour and resource intensive. It involves treatment for many women not destined to develop invasive cancer. The increased intervention rate for cervical abnormality in England is due to change in practice, not a cohort effect, and is probably the reason for the marked fall in incidence and mortality during the 1990s. For other cancers there is scope for major iatrogenic harm from screening because of invasive tests and treatments.

Information about screening - is it to achieve high uptake or to ensure informed choice?

For many years, public information about screening has been aimed at achieving high uptake but concerns are now being raised about this approach. There are several problems that have prompted these concerns. By giving information that emphasizes only the positive aspects of screening the autonomy of individuals is ignored, individuals feel angry when they perceive that they are let down by screening, symptoms may be disregarded because of the belief that screening gives full protection, health service staff carry the blame for problems that are in fact inherent in screening, and sound debate about policy and investment in screening is hampered by misunderstanding about the benefits and costs of screening. If we adopt instead an approach that makes explicit the limitations and adverse effects then a different set of problems will be encountered. We risk a reduction in uptake of screening and thus population benefits may reduce, those most likely to be deterred from accepting screening may be the most socially disadvantaged, there will be a cost in terms of staff time to explain screening more fully to participants, and cost-effectiveness could be reduced if uptake falls so low as to make services barely viable. In the UK current General Medical Council (GMC) advice to doctors about informed consent for screening makes it clear that full information should be given. The UK National Screening Committee has also signalled the need for a changed approach to information giving so that individuals are offered a choice based on appreciation of risks and benefits. It will take time for this approach to be fully reflected across the full range of UK screening programmes. New national information will be needed to assist staff in giving full information, and some aspects of policy, such as screening coverage targets for Health Authorities and General Practitioners, will need to be altered. There are many questions still to be answered about the kind of information needed to achieve informed participation, and about how it should be framed and communicated. These questions can begin to be addressed when there is clarity at national level about the purpose of information about screening.

Detection rates for abnormal cervical smears: what are we screening for?

We analysed rates of detection for smear abnormalities in 255,000 women served by the Bristol screening programme. The programme began in 1966 with the aim of eradicating the 30-40 deaths each year in Bristol from cervical cancer. Organisation has been good and population uptake has been high for the past 15 years. Records were computerised in 1977. During the 1988 to 1993 screening round, 225,974 women were tested. New smear abnormalities were found in 15,551, of whom nearly 6000 were referred for colposcopy. These numbers are excessively high in comparison with the incidence of the malignancy we are trying to prevent. The effect of screening on death rates in Bristol is too small to detect. Our conclusion is that despite good organisation of the service, much of our effort in Bristol is devoted to limiting the harm done to healthy women and to protecting our staff from litigation as cases of serious disease continue to occur. The real lesson from 30 years' cervical screening is that no matter how obvious the predicted benefit may seem for any screening test, introduction should never take place without adequate prior evaluation of both positive and negative effects in controlled trials.

Long term follow up of women with borderline cervical smear test results: effects of age and viral infection on progression to high grade dyskaryosis.

OBJECTIVE: To follow up and assess the significance of borderline change in cervical smears. DESIGN: Retrospective study of women undergoing routine cervical cytological screening in 1981. SETTING: Avon Cervical Screening Programme, covering 250,000 women in Bristol and Weston super Mare. SUBJECTS: 437 women showing borderline cervical changes in 1981 and 437 age matched controls with normal results in 1981. MAIN OUTCOME MEASURES: Cytological progression to high grade dyskaryosis (cervical intraepithelial neoplasia grade III or invasive carcinoma). RESULTS: During follow up ranging from 13 to 106 months 98 of the 437 women (22.4%) with borderline cytological changes on routine cervical cytology screening had a subsequent smear test showing high grade dyskaryosis compared with three of the 437 women (0.9%) in the control group. The risk of progression was greater in women aged 20 to 39 than in those aged 40 and over. Human papillomavirus infection had initially been diagnosed cytologically in 101 of the 437 (23%) women with borderline results. Significantly fewer of these women developed high grade dyskaryosis (13/98 (13%) v 88/339 (26%), p less than 0.05). CONCLUSIONS: Women with borderline smear test results are at increased risk of developing high grade dyskaryosis, particularly if the borderline changes occur without cytological features of human papillomavirus infection. Progression occurs within three years in 50% of cases, although a linearly increasing risk was sustained over the nine years of follow up and was greatest in women aged 20 to 39. Careful follow up of these women is indicated.