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NEW YORK (Reuters Health) - Patients asked to estimate how many lives would be saved through cancer screening or how many hip fractures can be prevented with bone-building medication mostly overestimate the benefits of these preventive measures, according to a survey of New Zealanders.
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 O'Connor, who was not involved in the new study.
Doctors, nurses and others 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, O'Connor told Reuters Health.
"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 and a professor at 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.
Hudson said that in talking with his patients about screening, he found they were surprised by how small the benefits were.
To get a broader sense of patients' expectations for preventive measures, 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 bone-strengthening medication.
Specifically, participants were asked to imagine scenarios in which 5,000 people between ages 50 and 70 undergo 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 seven percent of the participants answered in the correct range of one to five lives being saved with screening, whereas 90 percent 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, Hudson's group reports in the Annals of Family Medicine.
Eighty-two percent of participants overestimated the number of fractures prevented by bone-strengthening medication, which in reality is about 50 for every 5,000 patients given the drug.
And 69 percent of participants reported that 500 or more lives would be saved if 5,000 people took blood pressure medication, 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," Hudson told Reuters Health.
"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.
The U.S. Preventive Services Task Force (USPSTF) issues screening recommendations and other guidelines for disease prevention, which doctors, nurses and public health groups often consult when counseling patients.
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 (see Reuters Health story of March 5, 2012).
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 something called the "relative risk."
The relative risk describes the change in a person's chances of developing a disease, but it does not give any sense of how much risk that person had to begin with.
For example, a "50 percent reduction in risk" may be less significant than it sounds if a person's absolute risk for a condition - how likely they are to develop it at some point in life - was originally five percent, and drops to 2.5 percent.
"Professionals and people who provide health information need to know absolute benefits," O'Connor said.
The relative risk of dying from breast cancer is 17 percent lower among a population of women aged 50 to 69 who get screened, compared to women who do not get screened, for instance.
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.
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 Hudson.
Hudson advocated 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.
SOURCE: bit.ly/XINd5u Annals of Family Medicine, November/December 2012.