Cervical Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Description of the Evidence
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%.
One important factor in the accuracy of the Pap test is the adequacy of the specimen obtained. Adequate training and using techniques such as the cytobrush may improve sensitivity.
New Screening Technologies
Newer techniques that employ liquid-based cytology (e.g., ThinPrep) have been developed to improve the sensitivity of screening. As with the Pap test, the optimal studies to determine the sensitivity and specificity of these technologies have not been done. Some less-than-optimal studies show that sensitivity is modestly higher for detecting any degree of CIN, with modestly lower specificity.[32,33] One careful study, however, showed that conventional Pap testing was slightly more sensitive and specific than liquid-based cytology.