|Measure ||Desired Direction ||As of this Report |
|Three-year negative predictive value of Pap tests for Ontario screen-eligible women aged 21 to 69 || || |
| See Methodology and Approach to find out how the ratings are calculated. |
Among women who had a normal Pap test in 2010, the 3-year negative predictive value of the Pap test (i.e. the probability that a woman would be free of carcinoma in situ—abnormal cells in the cervix—within 3 years of a normal Pap test) was 99.9%. The negative predictive value of Pap tests was similar among women who had normal Pap tests in 2008 and 2009. The 3-year negative predictive value of a Pap test was high across all age groups.
What is cervical cancer screening (Pap test)?
- A screening test identifies people in a healthy, asymptomatic population who may be at risk for disease. It is not a diagnostic test.
- The purpose of cervical screening (Pap test) is to prevent cancer by identifying pre-cancerous changes to the cells of the cervix.
- Changes in the cervix usually develop very slowly over many years, so there is a long period of time when abnormal cell changes can be detected before cervical cancer appears. These changes, called precursor lesions, can be found with a Pap test.
- Colposcopy is a diagnostic procedure performed following an abnormal Pap test result. This procedure allows a colposcopist to selectively take tissue samples of cervical abnormalities so a pathologist can make a definitive diagnosis and a doctor can make a treatment plan1.
- After the diagnostic process is complete, a woman requiring treatment could undergo a loop electrical excision procedure, laser therapy, cryotherapy or cold knife cone biopsy1. Treating precursor lesions is intended to prevent the development of cervical cancer.
- Cancer Care Ontario updated its cervical cancer screening guidelines in 2012. It now recommends cervical cancer screening every 3 years for women aged 21 to 69 who are, or who have ever been, sexually active. Screening can stop at 70 years of age in women who have had 3 or more normal tests within the previous 10 years2.
What do the results show?
- The 3-year negative predictive value of Pap tests for women who had a normal Pap in 2010 was close to 100%, which is similar to the negative predictive value in previous years.
- A woman being diagnosed with invasive cervical cancer within 3 years of a normal Pap test was a rare event (data not shown).
- The 3-year negative predictive value of Pap tests was minimally lower in women aged 21 to 29 (99.6% in 2010). The same pattern was observed from 2008 to 2010.
- While the negative predictive value of Pap tests is high, they are not 100% accurate (i.e. some abnormalities may be missed by a Pap test). Therefore, it is important that eligible women are screened regularly at the intervals recommended in the guidelines so there are multiple opportunities to detect pre-cancerous changes in the cervix.
- Cervical screening according to Cancer Care Ontario’s guidelines is especially important in younger women (aged 21 to 29). In this age group, human papillomavirus (HPV) infections, which can cause cervical cancer, are likely to be transient and go away without treatment3.
- Despite the very high negative predictive value of Pap tests, 1,600 women in Ontario had a normal Pap test in 2010 and were diagnosed with carcinoma in situ within 36 months of their normal Pap test. Most of these women were aged 21 to 29. This is equivalent to 0.15% of the more than 1 million women who had a Pap test in 2010.
Why is this important to Ontarians?
- Virtually all cervical cancers and their precursor lesions are caused by persistent infection with high-risk oncogenic (i.e. cancer-causing) HPV types, especially types 16 and 184–6. HPV is transmitted through intimate sexual contact and is common among sexually active men and women7,8. Most sexually active men and women will acquire an HPV infection in their lifetime8–10. Most HPV infections will go away or clear without causing harm. Factors that determine clearance as opposed to persistence of a high-risk HPV infection are not well understood.
- Persistent high-risk HPV infections are a necessary factor in the development of cervical cancer and pre-cancerous changes in the cervix5,6.
- HPV vaccination programs offer a major reduction in the frequency of more common HPV viral infections. Since high-risk HPV infections rarely cause symptoms, screening is the only way to detect changes caused by high-risk HPV that might lead to cervical cancer precursors.
- The very low incidence (i.e. new cancer cases) of invasive cervical cancer and carcinoma in situ within 3 years of a normal Pap test supports the 3-year screening interval.
- Timely follow-up of abnormalities that are found through cervical screening is necessary for the reduction of cervical cancer incidence and mortality (i.e. deaths).
- Cervical cancer incidence has declined by as much as 80% where the screening quality, access and follow-up of women are high11. See the Special Focus story on cervical cancer for information regarding the decline in cervical cancer mortality in Ontario.
Cervical cancer screening needs to continue in an era of HPV vaccination.
- Vaccines are now available to prevent infection with the major oncogenic strains of HPV that have been linked to the development of the majority of cervical cancer cases12.
- Vaccinated women should continue to be regularly screened for cervical cancer. While current vaccines target the majority of oncogenic types, other oncogenic types exist.
- Moreover, vaccines do not provide protection for those infected with high-risk HPV before immunization.
- Because HPV infections and pre-cancerous cervical changes rarely cause symptoms, screening is the only way to detect early changes that could lead to cervical cancer.
- It is anticipated that there will be a significant reduction in the number of HPV infections due to widespread HPV immunization. The implications of HPV immunization on cervical screening will continue to be considered.
- Cancer Care Ontario is developing strategies to increase access, align processes, improve practices and enhance the quality of colposcopy services in the province.
- The Ontario Cervical Screening Program (OCSP) continues to pursue optimal screening strategies outlined in the 2011 Cervical Screening Guideline by the Program in Evidence-Based Care at Cancer Care Ontario, and to respond to changes in knowledge and screening strategies13.
- The OCSP will continue to explore the screening implications of an increase in proportion of HPV-immunized women in the screen-eligible cohort and is planning to address them.
- In order to improve cervical cancer prevention—the objective of cervical cancer screening—Cancer Care Ontario is establishing next steps for organizing colposcopy services in the province.
- Cancer Care Ontario [Internet]. Toronto: Cancer Care Ontario. Insight on cancer: news and information on cervical cancer; 2005 Oct. [cited 2015 Dec 18]. Available from: https://cancercare.on.ca/common/pages/UserFile.aspx?fileId=13802.
- Cancer Care Ontario [Internet]. Toronto: Cancer Care Ontario. Ontario cervical screening cytology guidelines summary; 2012 May [cited 2015 Dec 18]. Available from: https://cancercare.on.ca/common/pages/UserFile.aspx?fileId=13104.
- Ho GYF, Bierman R, Beardsley L, Chang CJ, Burk RD. Natural history of cervicovaginal papillomavirus infection in young women. N Engl J Med. 1998 Feb 12; 338(7):423–8.
- Kwong J, Crowcoft N, Campitelli MA, Ratnasingham S, Daneman N, Deeks SL, et al. Ontario burden of infectious disease study (ONBOIDS). Toronto: Ontario Agency for Health Protection and Promotion, Institute for Clinical Evaluative Sciences; 2010.
- Walboomers JM, Jacobs MV, Manos MM, Bosch FX, Kummer JA, Shah KV, et al. Human papillomavirus is a necessary cause of invasive cervical cancer worldwide. J Pathol. 1999 Sep; 189(1):12–9.
- Franco EL, Schlecht NF, Saslow D. The epidemiology of cervical cancer. The Cancer Journal. 2003 Oct; 9(5):348–59.
- Yabroff KR, Mandelblatt JS, Kerner JF. Effectiveness of interventions to improve follow-up after abnormal cervical cancer screening. Prev Med. 2000 Oct; 31(4):429–39.
- Brown DR, Shew ML, Qadadri B, Neptune N, Vargas M, Tu W, et al. A longitudinal study of genital human papillomavirus infection in a cohort of closely followed adolescent women. J Infect Dis. 2005 Jan 15; 191(2):182–92.
- Koutsky L. Epidemiology of genital human papillomavirus infection. Am J Med. 1997 May 5; 102(5A):3–8.
- Bosch FX, de Sanjosé S. 2003. Human papillomavirus and cervical cancer: burden and assessment of causality. J Natl Cancer Inst Monogr. 2003; (31):3–13.
- World Health Organization and the International Agency for Research on Cancer (IARC). IARC handbooks of cancer prevention: cervix cancer screening. Vol. 10. Lyon: IARC Press; 2005.
- Smith JS, Lindsay L, Hoots B, Keys J, Franceschi S, Winer R, et al. Human papillomavirus type distribution in invasive cervical cancer and high-grade cervical lesions: a meta-analysis update. Int J Cancer. 2007 Aug 1; 121(3):621–32.
- Murphy J, Kennedy S, Dunn M, Fung Kee Fung M, Gzik D, McLachlin CM et al. Cervical Screening [Internet]. Toronto: Cancer Care Ontario; 2011. [cited 2016 Mar9] Available from: https://cancercare.on.ca/common/pages/UserFile.aspx?fileId=124511