Cervical cancer mortality is about 40% higher in black women younger than 65 years than in white women of the same age. Among women older than 65 years, cervical cancer mortality for black women is more than 150% higher than for white women. In either case, mortality is rare among women of any age who have regular screenings.
The Pap Test
The Pap test has never been examined in a randomized controlled trial. A large body of consistent observational data, however, supports its effectiveness in reducing mortality from cervical cancer. Both incidence and mortality from cervical cancer have sharply decreased in a number of large populations following the introduction of well-run screening programs.[15,16,17,18] In Iceland, the mortality rate declined by 80% for more than 20 years, and in Finland and Sweden by 50% and 34%, respectively.[15,19] Similar reductions have been observed in large populations in the United States and Canada. Reductions in cervical cancer incidence and mortality were proportional to the intensity of screening.[15,19] Mortality in the Canadian provinces was reduced most remarkably in British Columbia, which had screening rates two to five times those of the other provinces.
Case-control studies have found that the risk of developing invasive cervical cancer is three to ten times greater in women who have not been screened.[21,22,23,24] Risk also increases with long duration following the last normal Pap test, or similarly, with decreasing frequency of screening.[25,26] Screening every 2 to 3 years, however, has not been found to increase significantly the risk of finding invasive cervical cancer above the risk expected with annual screening.[26,27]
Accuracy of the Pap Test
Ideally, determining the sensitivity and specificity of a screening test would involve a study that applies a "gold standard" test (such as colposcopy with appropriate biopsy) to all participants (whether the screening test is positive or negative). Sensitivity (the percentage of "true-positive" cases that are detected by the screening test) and specificity (the percentage of "true-negative" cases that are negative by the screening test) could be calculated. Such studies have rarely been done for any screening test for cervical cancer. Studies that compare the Pap test with repeat Pap testing have found that the sensitivity of any abnormality on a single test for detecting high-grade lesions is 55% to 80%.[28,29] Because of the usual slow-growing nature of cervical cancer, the sensitivity of a program of regular Pap testing is likely higher.
To determine the sensitivity and specificity of the Pap smear, both a test threshold (i.e., the point at which the test will be considered to be "positive") and a reference-standard threshold (i.e., the point at which the reference standard is considered to be "positive") must be defined. In practice, ASCUS is often used as the test threshold, and CIN 1 is often used as the reference threshold. This combination gives a sensitivity of about 68% and a specificity of about 75%. A more appropriate test threshold may be LSIL, with a reference threshold of CIN 2–3. This combination gives a sensitivity of 70% to 80%, with a specificity of about 95%.