By Michael Raj
The incidence of cervical cancer has been decreasing over the past three decades, largely due to routine screening. When screening leads to the detection of pre-cancerous lesions, they can easily be treated, and cancer can be avoided; and when screening finds cancer at an early stage, it can usually be treated successfully. Nevertheless, in 2017, it is estimated that there were 1,550 cervical cancer cases diagnosed and 380 deaths in Canada.
Current guidelines recommend screening every two to three years using a cytology-based test starting at age 21 through to ages 65 to 70, depending on the jurisdiction. The cytology test can be either a conventional Pap test (or Pap smear) or the newer liquid-based cytology test. Because both tests are commonly referred to as Pap tests, this term is used in this article to refer to both types of cytology. During a Pap test, a health care provider takes a swab of cells from the surface of the cervix, which is then examined under microscope for abnormalities. If a screening test comes back positive, the patient will then undergo a colposcopy to examine the cervix and determine if cancer is present.
Another test — the HPV or human papillomavirus test — can also be used for cervical cancer screening. Rather than looking for cell abnormalities, this test examines the cells for the presence of the DNA of high-risk (cancer-causing) types of the HPV virus.
HPV is the most common viral infection of the reproductive tract and is the major risk factor for the development of cervical cancer. Almost all cervical cancers are caused by HPV. While there’s currently no cure for HPV infection, it can be prevented with vaccination. School-based programs have been implemented in all Canadian provinces and territories with different starting ages and dosing schedules.
Most sexually active people will have an HPV infection at some point in their lives, and in most cases, the infection will clear on its own. But a persistent HPV infection can lead to changes in the cervical cells, which can develop into cancer. It could take as long as 10 to 15 years for these cells to become cancerous. It’s therefore important to find these cell changes early through regular screening.
This strong link between HPV infection and cervical cancer has caused many experts to suggest that HPV-based screening replace the Pap test as the primary screening test for cervical cancer. It’s thought that the HPV test might be better at correctly identifying those who are at risk of developing cervical cancer compared with the Pap test. Further, cell samples for HPV tests can also be collected by patients themselves, whereas Pap test samples can be collected only by clinicians. This may be particularly important for those who find the screening process embarrassing or invasive, or who don’t screen for other reasons.
Despite these potential advantages, HPV testing hasn’t been implemented in any Canadian provinces or territories yet, although a few are working on it. Should the Pap test be replaced by the HPV test? What does the evidence say?
To help guide decision-making, policy makers and the health care community turned to CADTH — an independent agency that finds, assesses, and summarizes the research on drugs, medical devices, tests, and procedures — to find out what the evidence says. CADTH undertook a health technology assessment to help answer questions about whether the HPV test should replace the Pap test in Canadian jurisdictions as the primary screening tool for cervical cancer.
The CADTH review found that there would be both benefits and drawbacks to the broad adoption of either type of test. Because the HPV test is very effective for detecting the types of HPV that put an individual at a high risk for cervical cancer, it may be able to identify a greater number of at-risk patients and, ultimately, result in fewer cases of cervical cancer being missed. However, because not all patients with high-risk HPV will go on to develop cervical cancer, the test would likely result in an increase in patients undergoing additional, more invasive testing only to find out that they don’t have the disease. It should be noted that CADTH found few long-term clinical studies comparing the HPV and Pap test, so it’s unclear if identifying more at-risk patients would, over time, lead to fewer deaths from cervical cancer.
Decision-makers thinking about a switch from Pap testing to an HPV-based screening program need to balance a wide range of considerations. The CADTH review assesses not only the comparative accuracy and clinical utility of the two screening tests but also their safety and cost-effectiveness, and it also explores the experiences and perspectives of patients as well as ethical and implementation issues. The reports produced from the review are freely accessible at www.cadth.ca/hpv-testing-primary-cervical-cancer-screening.
Michael Raj, BSc MHSA is a Knowledge Mobilization Consultant for CADTH’s Implementation Support and Knowledge Mobilization program.