Cancer

16 November 2012

Benefits of prevention overestimated

Members of the public who were asked to estimate how many lives would be saved through cancer screening.

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Members of the public who were asked to estimate how many lives would be saved through cancer screening or how many hip fractures could be prevented with bisphosphonates mostly overestimated the benefits of these preventive measures, in a survey of New Zealanders.

Dr Annette O'Connor of the University of Ottawa, who studies how patients weigh risk and make decisions, said she would expect that people would overvalue any given prevention effort.

"Most people would overestimate because they're told about their benefits, but with no numbers so why would you think that it's going to be really low?" said Dr O'Connor, who was not involved in the new study.

Providers who communicate health information often don't detail how much a given test or drug can help, but only say that people ought to have it, Dr O'Connor said.

Expectations too high

"I think it's led to more people taking part in screening or availing themselves of preventive medication than would have been the case if they were presented the information in more meaningful terms," said Dr Ben Hudson, the new study's lead author, from the University of Otago in Christchurch, New Zealand.

"I would also be concerned that it's led to people having over-heightened expectations of what these things can achieve, and that may lead to disappointment when the inevitable breast cancer happens despite screening," he added.

Dr Hudson said that in talking with his patients about screening, he found they were surprised by how small the benefits were.

How the study was done

To get a broader sense of patients' expectations for preventive measures, Dr Hudson and his colleagues asked 354 people about the benefits of breast cancer screening with mammography, bowel cancer screening with stool testing, taking antihypertension medication and taking bisphosphonates.

Specifically, participants were asked to imagine scenarios in which 5 000 people between ages 50 and 70 receive one of these preventive interventions for 10 years, then asked how many "events" the participants thought would be avoided as a result of the measure.

For three of the four interventions in the survey, the event to be avoided was death and in the case of bone drugs, it was hip fracture.

For breast cancer screening, only 7% of the participants answered in the correct range of one to five lives being saved with screening, whereas 90% overestimated how many lives would be saved. Fully a third thought that 1 000 deaths would be averted.

The numbers were similar for bowel cancer screening, which is thought to save five to 10 lives for every 5 000 people tested, Dr Hudson's group reported in the November/December issue of Annals of Family Medicine.

82% of participants overestimated the number of fractures prevented by bisphosphonates, which in reality is about 50 for every 5 000 patients given the drug.

And 69% of participants reported that 500 or more lives would be saved if 5 000 people took anti-hypertensives, when the correct range should have been 50 to 100.

"It's probably unreasonable to expect people to make an accurate guess at the absolute number (of lives saved or fractures prevented), but what we found was a consistent trend toward higher levels," said Dr Hudson.

"I don't think most patients are likely to have access to good numerical data presented in a simple and informative way. I think that's part of the problem here," he said.

Better informing

The U.S. Preventive Services Task Force (USPSTF) issues screening recommendations and other guidelines for disease prevention. Dr O'Connor said that when health care professionals repeat these guidelines to patients, they often don't include the numbers when talking about benefits or they only refer to the relative risk, rather than the absolute risk.

A survey of U.S. physicians found that most of them don't fully grasp what the numbers mean when it comes to cancer screening.

"Professionals and people who provide health information need to know absolute benefits," Dr O'Connor said.

For example, the relative risk of dying from breast cancer is 17% lower among a population of women aged 50 to 69 who get screened, compared to women who do not get screened. But in absolute terms, that means that instead of 23 in 100 000 women dying of breast cancer, screening would reduce that number to 19 in 100 000 women.

Dr Hudson said that one of the potential problems that can arise when people overvalue a test is that if recommendations are scaled back because of insufficient benefits, people get upset.

In 2009, for instance, the USPSTF changed its guidelines for regular mammograms from beginning at age 40 to beginning at 50, because the number of lives saved through screening during that extra decade of life was too small compared to the potential harms from the screening itself and follow-up procedures.

A survey of women at the time found that most of them considered the new guidelines to be "unsafe," at least in part because they feared that insurers would no longer cover screening for women in their 40s who wanted it.

"The other thing that happens when you have an established screening program for which people have heightened expectations, it becomes very politically difficult to make any changes insofar as recommending reduced access, even when the evidence is pretty convincing that the outcomes are better," said Dr Hudson.

He advocates for better informing patients of the benefits and harms of any preventive intervention. "I have a feeling this would all be easier if we could present (patients) with this information, trust them with their decisions and support them in doing so," he said.

(Reuters Health, November 2012)

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