Wednesday, 21 March 2012

"Smear Tests Prevent Cervical Cancer." Really?

There’s a delightful series of ads being run on New Zealand television encouraging women to get regular cervical smear tests done. The latest in these ads, titled “The Beach” is on air at the moment. It’s not on youtube yet, but you can watch it here.

These ads are produced for the National Screening Unit, under the auspices of the National Health Board of the Ministry of Health—in short, the New Zealand government. They’re the same folks who run the breast screening program. (See my earlier blog Are Mammograms Good for You?.)

I think the women in this ad are really sweet and really natural, and I enjoy watching the ad, but what bothers me about it is the tag line at the end: “A smear test every three years could stop you from getting cervical cancer.” Really? Are smear tests a method of cancer prevention these days? Gosh! And here I thought they were just a method of cancer detection! And there’s a difference between detection and prevention, isn’t there? Is this misleading advertising?

You don’t hear concerns regarding cervical screening as you sometimes do regarding breast screening, probably because although the procedure is somewhat invasive, there’s no radiation or other potentially harmful aspects to getting a cervical smear, and they can detect cervical cancers before a person is aware of their own illness. But I got to wondering just what sort of record cervical screening actually has. So I did a little investigating.

According to a summary of research studies on cervical smear tests provided by the Women’s Cancer Information Centre in California[i], cervical cancer rates in the US dropped after the introduction of routine cervical smear tests, and 50% of cervical cancer cases now occur in women who have never been screened. I don’t know if that really tells us much of anything unless we know how many women have regular pap tests.  If it’s 50%, then having the test doesn’t make a difference.

In the US, a country with over 300 million people in 2007—and although it is a rough estimate, presumably about half of those are women, so let’s say 150 million women,--just  4021 died of cervical cancer in 2007[ii], a little over 2 thousandths of one percent. The New Zealand National Screening website claims that without screening, 1 woman in 90 will develop cervical cancer, compared to 1 in 570 when smear tests are conducted every three years, although they don’t clarify how they arrived at these figures, which are footnoted as “estimates”. They also state that currently about 60 women each year in New Zealand die of cervical cancer (3 thousandths of one percent) but if untested, 1 woman in 200 will die of cervical cancer[iii]—about half a percent, based on half the population of 4 million. 

Research just reported today[iv] that compared cervical cancer rates between the US, where the average woman may have 20-30 pap smears in her lifetime, and the Netherlands where 7 smears over a lifetime are the norm (national guidelines there recommend tests ever five years between the ages of 30 and 60) showed no difference in cervical cancer mortality rate. The authors conclude US physicians overscreen.

For a cervical smear—or pap test—the doctor scrapes a few cells from a woman’s cervix with a swab. In the laboratory, that sample is brushed onto a slide and examined with a microscope for cells that appear abnormal. Because there is room for mistakes at all three stages of this process, accuracy rates for smear tests have a fairly high risk of error. According to Slowik’s 2011 article How Accurate are PapSmear Results?, 20-45% of results are false negatives (reporting that cells are normal when they are not), and a smaller percentage are false positives (reporting the presence of cancerous or pre-cancerous cells when cells are normal). The New Zealand National Screening website gives a 20% rate for false negatives, a figure also reported by McMeekin, McGonigle, and Vasilev.

Cervical cancer rates seem to be higher in some specific demographic groups including Maori, Pacific Islanders, and Asian women in New Zealand (see Benefits of Regular Screening), and according to the NHS Screening Service in the UK, those who have multiple sexual partners, those who come in contact with high-risk HVP viruses, those who are HIV-positive, those who take immunosuppressive drugs, and those who smoke. Having had multiple pregnancies also increases the risk. Since the development of cervical cancer is generally very slow, it most commonly appears in middle-aged and older women (average 50-55 years). Most screening programs discontinue screening after age 65-70.

 At the end of the day, it seems that the risk of developing cervical cancer is actually pretty low, but there is no harm in having regular pap smears. By regular, every 3-5 years seems to be adequate unless you have been assessed as ‘at risk’, and keeping in mind that the accuracy of the test for detecting cancer or pre-cancerous cells is only about 80%. 

So, do smear tests prevent cervical cancer? I think it is misleading to suggest that, but early diagnosis of pre-cancerous cells can lead to earlier treatment, and that might be something that saves your life. Besides, at the very least, it’s good for the health industry and keeps people employed, including—I’m thinking of the ads here—some very fine actors. 

[i] D. S. McMeekin, K. F. McGonigle, and S. A. Vasilev, 2000-2007. Cervical CancerPrevention: Cost-Effective Screening.
[ii] See Demographics of the United States for population numbers; Centers for Disease Control and Prevention for US cervical cancer vital statistics.
[iii] Benefitsof Regular Screening, National Screening Unit Website

No comments:

Post a Comment

Thank you for your feedback. Allow time for it to be posted.