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Decisions about screening can be more complex than those about treatment. Well designed decision aids could help patients understand the risks as well as the benefits
Patients should be able to participate in health decisions, if they so choose. Decision aids have been developed to help patients make informed decisions about treatment and have been found to improve knowledge, reduce indecision, and improve agreement between values and choice.1 However, in screening, where informed choice is not common practice,2 decision aids are controversial and their effect is currently unknown.3,4 Nevertheless, the complexities of decisions about screening mean that decision aids could be of immense value. In this paper we highlight eight issues that are critically important for the development and use of high quality decision aids about screening. Some of these issues arise because of fundamental differences between screening and treatment interventions; other issues flow from widespread uncritical support for screening, as illustrated by the storm over PSA (prostate specific antigen) screening for prostate cancer.5,6
Weighing the evidence for and against a screening intervention is a complex task for policy makers and is likely to prove challenging for individual consumers too. In its latest edition, the US Preventive Services Taskforce has rated screening interventions on a five point scale, based on an assessment of whether benefits outweigh harms (or vice versa) and on the extent to which evidence is available to make that assessment.7 Unfortunately, the evidence available is often insufficient to make a recommendation.7 Even when evidence is available, policy makers have to judge whether the benefits outweigh the harms, and this decision is at least partly driven by the values of the people making the policy or recommendation. Their values may not accord with those of the individuals who are subsequently offered the screening test, underscoring the need for evidence based information to be presented to consumers.
To further complicate decision making, screening is implemented in various ways in different countries, on the basis of variable evidence, and with variable government regulation and protection for consumers. For example, evidence from randomised trials supports blood pressure screening,7 yet improved blood pressure screening and management through computerised systems is variably implemented.8 In contrast, cervical cancer screening programmes have been established within carefully regulated systems in many countries without evidence from randomised trials.7 Health authorities may also hold opposing views on whether screening is worthwhile—as, for example, in regard to prostate cancer.7,9 Finally, some screening (for example, by total body computed tomography) is available (and heavily marketed), even though there is no evidence that benefit outweighs harm, no endorsement from any health authority, and little regulation or protection for consumers.66
Thus we believe decision aids for consumers are urgently needed (and will be developed) on the substantial number of screening interventions now available. Below, we use examples from a range of screening programmes to illustrate the issues relating to development and use of decision aids (box).
People making treatment decisions have a disease for which they seek treatment. Screening, however, may uncover inconsequential disease (pseudodisease) that will never produce symptoms.10,11 Detection of inconsequential disease results in unnecessary diagnosis and treatment.6,10,12 This problem is most clearly recognised in PSA screening for prostate cancer13 but applies generally. For example, screening mammography results in overdetection of both invasive breast cancer14 and ductal carcinoma in situ,15 so although screening may reduce the risk of dying from breast cancer, it increases the chance of a breast cancer diagnosis.
The problem of inconsequential disease extends to screening for risk factors. For example, people identified as having high cholesterol concentrations or high blood pressure suffer immediate “labelling” effects16,17 and may view these conditions as diseases. In reality, however, many people with these risk factors will not develop symptomatic cardiovascular disease for many years, if ever.18
Inconsequential disease is considered by some to be the biggest drawback of screening.6,12 Therefore it is of concern that the general public are poorly aware of it.6 Decision aids must contain clear, numerical information on the chances of having inconsequential disease found and treated.
Decision aids for treatments present information about several possible options and help people weigh these up. Screening, on the other hand, involves a longer, more complex process, including the initial test plus the follow up investigations and treatment. Informed decision making requires a consumer to understand the whole process before making a decision, yet few people realise the consequences of a positive screening result until they face that abnormal result. For example, few women who participate in cervical screening know much about the follow up tests and examinations for an abnormal smear test result, and knowledge levels are low, even in those who undergo colposcopy and biopsy.19,20 Thus developers of screening decision aids must include information about the whole screening process (including follow up tests and treatments).
Considerations in developing decision aids for screening interventions—and suggestions to developers
In the treatment context, people are sick and treatment may result in improvement in the near future. Thus an immediate treatment benefit may need to be traded off against immediate (or perhaps delayed) side effects. In screening, the benefits are usually to be experienced in the future, possibly quite far into the future, but the harms can be immediate. Thus immediate harms must be traded against an uncertain future benefit. For example, screening can reduce the risk of death from bowel cancer by about 23% if people attend screening regularly every two years, for 10 years.21 This possible benefit over the next 10 years has to be traded against the risk of a perforated bowel as a result of a colonoscopy, which could occur after the first or any subsequent screening test.21 To present balanced information about benefits and harms, therefore, decision aids need to present information about the cumulative risk of benefits and harms (such as the cumulative risk of a false positive result, or requiring a biopsy22) over an appropriate time frame (most likely 10 years or more).
The numbers needed to treat associated with most treatments for symptomatic conditions are relatively small.23 In contrast, the benefits of screening are relatively rare, and the numbers needed to screen are large. For example, the number of men who need to be screened for raised cholesterol concentration to prevent one death from heart disease within five years range from 21 067 for men aged 25-34 to 231 for men aged 55-64, and the numbers are greater for women.18 For cervical screening, an estimated 1000 women need to be screened for 35 years to prevent one death from cervical cancer.24
Conveying very low event rates is technically difficult. The cumulative deaths over 10 years from bowel cancer for Australian men in their 60s are about 98/10 000 without screening and about 75/10 000 with screening.21 Thus the diagrams used to aid screening decisions may need to use 1000 faces or even 10 000 faces to show event rates as whole figures instead of the 100 faces used in diagrams for treatments.25 Furthermore, it is important to provide a context within which to consider the impact of screening.26 For example, the decision aid might include the risk of death from other causes or all causes over a similar time.
Research indicates that people vary in the way they value screening outcomes such as false positive results.6,27,28 This is consistent with qualitative reports we have received from participants in bowel, breast, and cervical screening programmes: some people consider a false positive a good result because they feel reassured that they have been thoroughly checked and found not to have the target disorder, while others regard a false positive as inconvenient, anxiety provoking, unpleasant, or dangerous. Thus the information presented by and values clarification exercises used in screening decision aids need to be flexible about classifying outcomes as pros and cons. The aid might allow people to assign positive (pro) or negative (con) status to different outcomes themselves, rather than use a fixed scheme for everyone.
For treatments, decision aids use evidence from randomised trials about outcomes of different options. Such high quality data are often not available for screening, partly because trials of screening need to be very large and may require follow up over many years, making them expensive and time consuming. Worse, by the time the trial reports, the screening test may have been substantially updated, so the applicability of the results of the trial is unclear. Thus developers may have to rely on observational data with all its inherent biases (including lead time bias, length time bias, and the healthy volunteer effect10,11), which should be explicitly stated.
Even when trials are available, there are usually only a few, and so estimates are subject to substantial uncertainty. For example, meta-analysis of trials of screening for bowel cancer using faecal occult blood tests shows a 23% relative risk reduction for death from bowel cancer for people who screen regularly, with a 95% confidence interval of 11% to 43%.21 There may be even greater uncertainty about estimates from randomised trials in other contexts—for example, mammography screening.29 It is challenging to adequately convey the uncertainty in the data about screening. Ranges may be better than point estimates, but their use may compromise the simplicity and clarity of communication.
Consumers are reasonably familiar with having different treatment options to choose between, as there have been different drugs or management options available for many conditions for a long time. The concept of having options with regard to screening is less familiar.6 Developers of decision aids for screening should be prepared for users to respond with “But why wouldn't I be screened for X? That's the responsible thing to do.” We have found it necessary to explain the down sides of screening and reasons why you might choose not to be screened at the start of decision aids.
Furthermore, some healthcare providers may feel uncomfortable with the information provided in evidence based decision aids about screening and may disagree with consumers about the amount and balance of information. When pilot testing a mammography decision aid we found that healthcare providers thought it contained too much information and was slanted against screening, while women said the information was about the right length and slanted towards screening (D Stacey et al. Reasons for Hope, Ottawa, 25-27 Oct 2003). Our experience is consistent with previous research showing ultrasonographers felt they couldn't give women information contained in an evidence based leaflet about antenatal screening.30 Developers should probably expect some discomfort among healthcare providers, who may feel that the decision aid is unbalanced, that it runs counter to their view of screening, or that information about harms will cause anxiety among patients.
The effects of decision aids about screening on decision quality and participation in screening are largely unknown
Public (and professional) understanding of the drawbacks of screening is limited
As well as information on benefits, decision aids need to include information on detection of inconsequential disease and about the risks of the full range of investigations and treatments which may be entailed if results are abnormal
Information on benefits and risks should be given over similar time frames
People may vary in viewing outcomes of screening as pros or as cons
Financial incentives or performance targets are rarely related to acceptance of treatment. In the screening context, companies directly market “off the shelf” screening kits such as faecal occult blood test and genetic screening kits. Unlike treatments, screening tests are not well regulated by government, and the public is not well protected against the commercial promotion of screening tests.6 Even within authorised and accredited government funded screening programmes, there is a perceived need to meet participation targets in order to maximise population benefits.2 Healthcare funders and those delivering services may be reluctant to support the use of screening decision aids by consumers if they think it will reduce participation. The impact of screening decision aids on participation rates is unclear,3,4 but is likely to vary between different screening interventions. Developers may need to work closely with providers of the screening service to develop and evaluate screening decision aids. Any vested interest should be declared.
Screening decisions are complex, and relevant information, such as information about the increased risk of diagnosis with screening, is often counterintuitive. Benefits are relatively rare and often delayed, and the screening process can involve a whole series of interventions. The harms of screening are poorly understood by the public, and screening tests are often viewed uncritically. As such, screening decisions generate unique issues for consideration in the development and implementation of decision aids. Developers of screening decision aids need to ensure these issues are adequately addressed.
Contributors and sources: The ideas discussed arose through our joint work developing and evaluating decision aids for cancer screening. All authors contributed ideas, experience, and suggestions. AB wrote the first draft and all authors critically reviewed it and contributed to subsequent drafts. All authors are guarantors.
Competing interests: None declared.