Cervical screening is one of the most effective screening tests, responsible for substantial reduction in death from cervical cancer. This disease used to affect two per cent of Canadian women and kill one per cent.

But as with so many good things, the procedure can be overused, and Canadian doctors are overscreening their female patients.

It is time to change cervical screening policy: Start at a later age, and do it less often. Canadian provinces used to start testing from a woman’s first sexual activity. In 2011, Alberta changed this to start screening at age 21, and other provinces followed suit.

After examining the evidence, I maintain we should start even later — at age 25. Alberta and British Columbia have made this change; other provinces are considering it.

Screening under age 25 has little impact

Cervical cancer is largely a late result of an earlier infection with human papilloma virus (HPV) — a wart virus. It can develop several years after women become sexually active but mainly shows up when women are in their 40s or older.

I was a member of a team of epidemiologists and a gynecologist who examined the Canadian evidence from before screening started. We found that there was almost no cervical cancer in women under age 25 and very little under age 30. For many years, Canadian doctors performed pap tests when they prescribed contraception, so young women were tested annually. Yet this made no measurable difference to the numbers of invasive cancers.

In Britain, a large case-control study found that screening women aged 20 to 24 had little or no impact on rates of invasive cervical cancer up to age 30. Sadly, the few women who do get cervical cancer at these ages have rapidly advancing disease, and neither screening nor treatment seems to help much.

Screening causes harm

If excess screening was only an unpleasant nuisance for women, it might be acceptable, but there is more: It causes harm.

When young women are infected with vaginal warts, it is difficult to distinguish the cellular changes from cancer. Consequently, more than 10 per cent of women under 30 have “abnormal” pap tests, but this rate drops for older women.

A positive screening test causes anxiety for many women, though HPV warts often go away on their own by the time a repeat test is done.

If the tests appear abnormal, women are referred for further examination by colposcopy (when the cervix is examined under magnification, usually by a gynecologist) and small samples (biopsies) are removed for microscopic examination.

Some biopsies appear abnormal — a “pre-cancer” — and such women may have a larger “cone biopsy” or “loop electrosurgical excision” taken to remove the abnormal area at the entrance of the cervix. This “cures” the pre-cancer, but leaves a weakened cervix. Even though most abnormalities would resolve themselves, a few would not, so doctors treat them all.

The cervix, however, has an important biological role: It holds in babies until the time comes to release them. Women who have had a cone biopsy have a higher rate of pre-term births, and some of these babies die. Having a weakened cervix may not matter for older women who are no longer having babies, but for young women who want to birth children, this is a serious potential harm.

Balancing harms and benefits

What this means is that cervical screening policies and guidelines must balance the potential benefits against the harms of such screening, and recognize that the probabilities change with age.

The Canadian Task Force on Preventive Health Care examined the evidence, and in 2013 recommended that there should be no screening under age 20. Screening should start some time in a woman’s mid-20s, with an interval of every three years. Alberta and British Columbia changed their policies in accordance with the science.

Other provinces still start at age 21, and some still screen every two years. The reasons are not clear. Too often Canadians look south to the United States, where early frequent screening persists.

We might do better to check the rest of the world, where few countries start before 25, and some not until age 30.

Women should decide

If women were fully informed, how many would choose to have cervical screening so early and so often? They should have the choice, rather than being given uninformative positive encouragement of the type that’s found on many provincial websites.

Physicians need better information to share with their patients, and should not routinely perform cervical screening tests on young women at least until their mid-20s.

Women who start sexual activity later in life can wait even longer, since the earliest cancers do not develop until at least four years after first sexual activity, and mostly not until 10 to 20 years later. For those who have been immunized against HPV, the probabilities will be even lower.

Some doctors argue that women should still get regular pelvic examinations and test for sexually transmitted infections (STI). But pelvic examinations look for an enlarged uterus or ovarian disease, which mainly affect older women. And urine tests detect STIs. So doing routine pelvic examinations is unnecessary.

In provinces that do not have screening registries that remind women to get examined every three years, they may neglect to do so. So some physicians argue women should be told to come at two-year intervals to ensure they come within three years.

This means that compliant women will be screened more often than necessary, and subjected to extra risk of harm, but changes nothing for those who do not attend.

Instead, women and their doctors should follow the science, not inappropriate policies, and women should decide for themselves what is right for them.