The Pap smear revisited.

Papanicolaou smear screening for cervical cancer has become an established practice in most developed countries. This is because the cervix is relatively accessible to investigation and treatment, and early stages in the morphogenesis of cervical cancer are both recognizable and easily treated. The Pap smear is a valid test. It is simple, relatively inexpensive, reliable, and free of risk. Although the test has far from perfect sensitivity, it has high specificity, and false-positive results are rare. In most reported series, the majority of false-negative results have been found to be attributable to collection errors rather than laboratory errors. Despite the importance of Pap smear screening, controlled prospective trials have not been undertaken to determine its efficiency in reducing cervical cancer incidence and mortality. However, countries with well-organized programmes, wide population coverage and correct follow-up appear to have had some impact on mortality from cervical cancer. Nevertheless, coverage of high-risk groups, particularly women over 40 years of age, remains the greatest problem. Recommendations on the frequency of testing vary considerably. Statistical models indicate triennial testing may deliver almost all of the effectiveness of annual testing at a substantially reduced cost, but the numerous reports of false-negative results argue strongly in favour of annual screening. It is possible that these problems may be solved in the future by increasing the sensitivity of the test and/or by the use of additional tests.