Working Procedures

B. SCIENTIFIC REVIEW AND EVALUATION

5. Effectiveness of population–based screening

The impact of the screening procedure when implemented in defined populations is examined in this section. Indicators used to monitor effectiveness, such as positive and negative predictive values, detection rate, rates of interval cancers, and the number of tests performed, are reported. Time trends before and after implementation of screening as well as comparisons, including geographical comparisons, of the occurrence of the disease and death from the disease in populations exposed and not exposed to screening are reviewed and interpreted. In doing this, the Working Group takes into account differences in screening procedures (e.g. frequency and the age of the target population) and of participation rates.

An integral component of this section is an evaluation of the expected benefit or harm of the screening procedure to the population. Reductions in all-cause mortality, mortality from the disease and/or the incidence of invasive disease are fundamental measures of benefit. An additional benefit is that more cases may be treated by less aggressive, less invasive procedures, thus improving the quality of life.

The spectrum of health care is dynamic, and a screening procedure should not be viewed in isolation. Greater awareness of the disease, brought about by publicity about screening that may result in early diagnosis, could be regarded as another benefit of a screening programme. Also, in this section the possibility should be considered that there might have been a change in treatment of the cancer, which even in the absence of screening would have resulted in a substantial decrease in mortality. As far as possible, an evaluation should be made of the extent to which improved treatment has been responsible for any changes seen in mortality from the specific disease. Estimates of rates of false-positive and false-negative findings in screened individuals and their consequences (false sense of security with false-negatives, and false alarm with false-positives) are an integral part of this section. The rates of short- and long-term side-effects and the possibility of unnecessary treatment of borderline or indolent cases detected at screening are discussed.

Management procedures for lesions detected at screening are reviewed. Psychological factors, such as anxiety induced by undergoing the test procedure, are also considered. Finally, the cost-effectiveness of various modalities of test administration in various settings is considered. The discussion takes into account the costs per case detected and per death prevented.

 

Posted 27 May 2014