The Canadian Cancer Society and the Canadian Task Force on Cervical Screening recommend that a Pap smear be done for all women by age 18 or who are sexually active, regardless of age. Women who have multiple sexual partners should be screened annually, but those in long-term, monogamous relationships who have had negative Pap smears two consecutive years may be screened every three years until age 65.
    One of the problems with the Pap smear is that it can be normal when there is a malignancy present. The pathologist examining the cells can make an error, the health care provider may not sample the cervix adequately or an infection might obscure the results. The estimated false negative rate is about 10 to 20 percent, half of which can be attributed to faulty sampling techniques.
    Adenocarcinomas and adenosquamous carcinomas are also more difficult to detect on Pap smears since they start high up in the cervical canal. It is, therefore, now recommended that Pap smears include a sample from the endocervix using an endocervical brush.