Effectiveness
Since the introduction of the Pap test, deaths caused by carcinoma of the cervix have been reduced by up to 99% in some populations wherein women are screened regularly. A regular program of pap smear screening, with appropriate follow-up, can reduce cervical cancer incidence by up to 80%.
Failure of prevention of cancer by the Pap test can occur for many reasons, including not getting regular screening, lack of appropriate follow up of abnormal results, and sampling and interpretation errors. In the US, over half of all invasive cancers occur in women that have never had a Pap smear; an additional 10 to 20% of cancers occur in women that have not had a Pap smear in the preceding five years. About one-quarter of US cervical cancers were in women that had an abnormal Pap smear, but did not get appropriate follow-up (woman did not return for care, or clinician did not perform recommended tests or treatment).
Adenocarcinoma of the cervix has not been shown to be prevented by Pap tests. In the UK, which has a Pap smear screening program, Adenocarcinoma accounts for about 15% of all cervical cancers
Estimates of the effectiveness of the United Kingdom's call and recall system vary widely, but it may prevent about 700 deaths per year in the UK. A medical practitioner performing 200 tests each year would prevent a death once in 38 years, while seeing 152 women with abnormal results, referring 79 for investigation, obtaining 53 abnormal biopsy results, and seeing 17 persisting abnormalities lasting longer than two years. At least one woman during the 38 years would die from cervical cancer despite being screened.
Since the population of the UK is about 61 million, the maximum number of women who could be receiving Pap smears in the UK is around 15 million to 20 million (eliminating the percentage of the population under 20 and over 65). This would indicate that the use of Pap smear screening in the UK saves the life of 1 person for every approximately 20,000 people tested (assuming 15,000,000 are being tested yearly). If only 10,000,000 are actually tested each year, then it would save the life of 1 person for every approximately 15,000 people tested.
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