Chapter 9 Screening

Screening

After reading this chapter you will be able to:

Linking these topics to the Medical Council exam objectives, especially section 78-3.

To screen or not to screen

Paul Richards consults Dr. Rao because his back has been causing him pain recently and he wonders if he needs an X-ray. In passing, he heard about the “prostate test” on the radio and he wants to know if he should get one. After taking a history and examining his back, Dr. Rao advises Paul to continue his normal activities and, if possible, to get more exercise. He does not find any indication for imaging and explains this to Paul. He checks the clock and wonders if he has time to discuss prostate screening or if he should ask Paul to arrange another appointment to do so…

Goal of Screening

Screening aims to detect disease in its early stages – before it becomes clinically manifest. As such, it is generally classified as secondary prevention (See Chapter 4). In formal terms it is the application of a test or procedure to asymptomatic people in order to classify them into two groups: one at high risk and the other at average risk of developing symptomatic disease. People in the high risk group undergo further investigation to identify those who have the disease. These are then -treated to prevent the effects of the disease.

The logic of screening assumes that the earlier a disease is diagnosed the better it will respond to treatment, resulting in longer survival and better quality of life for patients. But for all the simplicity of the idea, whether or not to screen has to be considered very carefully. Wilson and Jungner commented, “the central idea of early disease detection and treatment is essentially simple. However, the path to its successful achievement (on the one hand, bringing to treatment those with previously undetected disease, and, on the other, avoiding harm to those persons not in need of treatment) is far from simple though sometimes it may appear deceptively easy.”1 First, the screening test may not be perfectly accurate, false positives may be found or cases may be missed (see Figure 9.1); SENSITIVITY and SPECIFICITY were explained in Chapter 6. Second, not all people with early signs will develop clinical disease; this is discussed below. Hence, screening represents an intervention on a patient who, as he may not have the disease being screened for, may not benefit. The clinician has a responsibility to ensure that the risk of harm is very small. Growing awareness of the limits of screening has resulted in the elimination of several tests that were formerly routine (see Nerd’s corner box) and greater rigour in evaluating proposed screening activities.

Screening failures

Here is a list of once widespread screening tests that have been discarded because they do not meet the criteria for a useful screening test.

  • Urine dipsticks for renal disease – very poor specificity.
  • Wassermann test (for syphilis) before marriage – low incidence in the general population.
  • Regular electrocardiograms – poor predictor of cardiac events, and harm can arise from investigations of positive screens.
  • Homocysteine for heart disease – poor predictor of disease.
  • CA 125 for ovarian cancer – not specific and no effect on mortality.
  • Regular chest x-rays for lung cancer – not sensitive or specific.
  • Breast self-examination – no impact on mortality.
  • Self-examination for testicular cancer – positive predictive value unknown, low incidence, and good response to treatment.
  • Infrared temperature sensors at airports for SARS – temperature results unreliable as they are influenced by many other factors.
Figure 9.1 Outcomes of screening

Clinicians most commonly screen patients during routine office visits, based on guidelines for the tests appropriate for the patient’s age and sex; some examples were given in Chapter 4. Screening can also be focused on patients at high risk due to their previous medical history, taking the form of more active case finding. Screening may also be applied by public health agencies to segments of the whole population: mass screening (see Definitions box).

Screening terminology

Routine screening is applied to patients according to their age and sex, based on evidence that the screening test accurately identifies disease and its application leads to interventions that do more good than harm. The evidence supporting each test is reviewed by the Canadian Task Force on Preventive Health Care. 2 This type of population based screening may occur when individual clinicians respond to clinical practice guidelines. They can systematically screen in their practice population, for instance by implementing an office system, as described in Chapter 8, to ensure that all the practice’s adult patients have regular blood pressure checks. They can also screen opportunistically using any consultation as an opportunity to carry out appropriate screening interventions.

Case-finding also aims to diagnose pathological processes in the asymptomatic stage, but the population targeted is usually defined by risk factors other than demographic. Examples of case finding would be checking cholesterol levels in family members of a patient with familial hypercholesterolemia, or checking for retinopathy in patients with diabetes mellitus. Of course, case-finding should also be carried out only when the expected benefits outweigh the risks.

Note that the term case-finding is also used:

  1. In infectious disease control to mean a standard procedure whereby diligent efforts are made to locate and treat persons who have had close or intimate contact with a known case. This is also known as contact tracing,
  2. In epidemic control to mean seeking people who have been exposed to a risk or to potentially harmful factors and who have the disease. 3

Mass or universal screening is aimed at a population defined demographically. Examples would be the provincial newborn screening programmes or the Canadian breast cancer screening programme.

Criteria for Introducing a Screening Test

Given screening’s superficial attractiveness but underlying potential for wasteful, even harmful, use of resources, in 1968 the WHO published criteria for judging screening programmes developed by Wilson and Jungner.1 As well as evaluating the test characteristics, the criteria consider the disease characteristics and the organizational context of the screening. These criteria can also help clinicians decide whether or not to apply a screening test in their practice.

Note that the difference between a screening and a diagnostic test is subtle; it can be defined in terms of difference in the risk of the disease for which a patient is being screened or tested. In screening, the person is asymptomatic and at apparently average population risk. In diagnosis, the patient has symptoms or signs indicating a higher risk of the disease being tested for. Many of the test, patient and system characteristics in the WHO criteria may therefore also be applied to diagnostic testing.

Wilson and Jungner’s criteria for screening

  1. In relation to the disease:
  • The condition sought should be an important health problem.
  • The natural history of the condition, including development from latent to declared disease, should be adequately understood
  • There should be a recognizable latent or early symptomatic stage
  • There should be an accepted treatment for patients with recognized disease
  1. In relation to the test:
  • There should be a suitable test or examination.
  • The test should be acceptable to the population
  1. In relation to the health care system:
  • There should be an agreed policy on whom to treat as patients
  • Facilities for diagnosis and treatment should be available
  • The cost of screening (including diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole
  • Screening should be a continuing process and not a “once and for all” project

Each criterion forms part of a chain, which, like any chain, is only as strong as its weakest link. If the proposed screening activity fails to meet any one of the criteria, it should not be carried out.

The disease

The condition should be an important health problem

The term “important” relates to its severity, including its complications, but also to the frequency of the disease in the population. Screening for the common cold would not make sense as it does little harm to the sufferer. Screening for a rare disease also runs into problems: large numbers of people will undergo the potential harms of screening in order to find a single case. Moreover, the POSITIVE PREDICTIVE VALUE of a screening test diminishes with the frequency of the disease in the population so that the rarer the disease, the more people who undergo diagnostic testing (with its attendant stress and risks) will eventually be found not to have the disease after all.

Screening programmes may target the sections of the population most at risk to improve the positive predictive value and thereby improve the programme’s benefit to cost ratio. For instance, breast cancer screening is aimed at women from 50 to 75 years old even though breast cancer can occur in both sexes and much earlier.

When screening is not worthwhile

Chinese women in Hong Kong have a much lower incidence of breast cancer than women in western countries; therefore, a mammography programme there would currently bring little benefit.4 However, because Hong Kong lifestyles are changing, screening may become beneficial in the future.5Screening for cervical cancer does not start until age 25, since below that age cancer is extremely rare, and minor changes related to Human Papillomavirus which can give rise to abnormal results, commonly regress spontaneously.

The natural history of the condition, including development from latent to manifest disease, should be adequately understood

Chapter 1 presented a simplified idea of natural history as though disease progresses steadily, but this is not always the case. Disease may regress because of the patient’s natural defences. Other conditions may be indolent and will never progress enough to harm the patient. A current challenge in prostate screening is distinguishing rapidly progressing cancers that need immediate treatment from slowly progressing ones that will likely never need treatment (see the Illustration box). These variations may distort screening results in a way termed ‘length bias’.

Screening and overdiagnosis

About two thirds of cases of mild cervical dysplasia regress to normal, whereas progression from mild to severe dysplasia or worse occurs at a rate of about 1% per year.6 Slow progression may mean that a patient may die from other causes before succumbing to the cervical cancer detected by the screening.

The old adage that “more men die with prostate cancer than die of prostate cancer” is true. The prostate specific antigen (PSA) test is only about 40% sensitive but is still useful in detecting early cases of prostate cancer.7 However, autopsy series find that approximately 30%–40% of men over the age of fifty have prostate cancer—a proportion that rises with age—yet only about 3% of men die from it. The best way to manage early prostate cancer is still uncertain. Read the opinion of Prof Richard J. Ablin, who developed the PSA test, on how the test is used.

Studies of breast cancer screening have shown that up to 30% more breast cancers are diagnosed in the screened group than in the unscreened group, probably due to the detection of early cancers that would never otherwise have become apparent.8

Length bias

Variability in the natural history of a disease complicates the evaluation of a screening programme. Slowly progressing variants of disease remain in a pre-symptomatic (but screen-detectable) stage for longer than rapidly progressing variants of the same disease. Because of their longer duration, intermittent screening is more likely to detect slowly progressing variants, which have longer survival after the eventual appearance of symptoms. Therefore, a screening programme will tend to identify a disproportionate number of slow-progressing cases. If the programme is then evaluated based on survival time in this biased sample a falsely positive conclusion will be reached: see Figure 9.3.

Figure 9.2: Length bias. In this population there are equal numbers of slow (blue) and rapidly-progressing (red) cases. But the screening test will identify five slower cases and only two rapidly progressing ones. Calculating mean survival from these seven cases will give an impression of longer average survival than occurs in the population.
Figure 9.2: Length bias. In this population there are equal numbers of slow (blue)
and rapidly-progressing (red) cases. But the screening test will identify five slower cases
and only two rapidly progressing ones. Calculating mean survival from these seven cases
will give an impression of longer average survival than occurs in the population.

There should be a recognizable latent or early symptomatic stage

If screening is to detect disease in asymptomatic people then there must be an adequate time between the stage when it is detectable by screening and the stage when it causes symptoms. Successful early identification of rapidly progressive disease would require frequent screening which could be intolerable for the health system and the target population. Diseases with long latent periods are more amenable to early detection.

Pre-disease

The term “pre-disease” has been used to describe the early stage or latent phase when a disease is amenable to detection by screening.9 An example would be nuclear dysplasia that may eventually become a cancer. In principle, intervening in the pre-disease phase should arrest the development of disease. Removing a pre-cancerous lesion should prevent the onset of cancer, although many pre-cancers regress without any intervention.

Other examples of pre-disease are “pre-diabetes” and “pre-hypertension”. At the time of writing, diabetes is diagnosed and treatment initiated at a fasting blood glucose of 7 mmol/l or more (or > 11.1 mmol/l two hours post prandial or HbA1c of 6.5%).10 This level is generally below the level at which symptoms (polydipsia, polyuria and weight loss occur) and far below the level for diabetic ketoacidosis. It is chosen because population-level studies show that it is the threshold for an increasing risk of cardiovascular events and microvascular disease.11 So, in practice, we are treating diabetes in order to prevent cardiovascular and microvascular disease; we do not affect the progress of the diabetes itself. There is little than can be done to delay or reduce the onset of type 1 diabetes. In type 2 diabetes, some authors point out that treating prediabetes may delay the onset of diabetes by 3 to 4 years.12 However, not all untreated pre-diabetics progress to diabetes. Nonetheless, impaired glucose tolerance should be the trigger to modifying lifestyles to reduce weight and increase physical activity, and even to using pharmacotherapy.10 Hence, for some, pre-diabetes and diabetes merit much the same treatment.

Essential hypertension is another condition in which defining threshold levels on a continuous scale as cut-points between pre-disease and disease may lead to over treatment. Hypertension manifests itself only by its effect on end organs, increasing the risk of renal damage, heart failure and stroke. We try to prevent end organ damage using lifestyle counselling and antihypertensives to reduce the blood pressure. Patients on antihypertensive medications are likely to experience side effects, so may feel worse on treatment than off it.

Diagnosing pre-diabetes and pre-hypertension may lead to treating risk factors (pre-diabetes, pre-hyptension) for diabetes or hypertension, which are themselves risk factors for microvascular, cardiovascular, renal and cerebrovascular disease, with little evidence for improved outcomes.9

Lead-time bias

Long latent periods may also be a source of an error in evaluating screening known as lead-time bias. This arises from the delay between when disease is detectable by screening and when it produces symptoms and would be diagnosed without screening. Screening leads to an earlier diagnosis, as shown in Figure 9.3, so the patient lives longer with the diagnosis, but may not live longer overall. Hence, evaluating a screening programme in terms of survival time after diagnosis gives a falsely positive impression of success. Controlled trials of screening are the only way of eliminating lead-time bias.

Figure 9.3: Lead time bias
Figure 9.3: Lead time bias

A final problem in evaluating screening programs is that people who readily adopt new, supposedly healthy behaviours tend to be healthier than those who do not. Observational studies that compare people who chose to get screened with others who did not, generally have a greater proportion of this healthier group in the screened group; they therefore tend to show that people who opted for screening have better outcomes than those who did not. For instance, women who have Pap smears are less likely to die from cervical cancer. However, they also tend to be better-educated and wealthier and, therefore, to have a lower base-line risk of dying from cervical cancer than those who don’t get Pap smears. Even after controlling for stage at diagnosis, women in higher socio-economic groups have a better prognosis than those in lower socio-economic groups. It is difficult to determine how much the favourable outcome in women who get screened is due to the test and treatment, and how much is due to their other favourable health determinants.

There should be an accepted treatment for patients with recognized disease

If nothing can be done to reduce the impact of a disease, screening increases morbidity by lengthening the time the patient is ill. The asymptomatic person becomes a patient, assumes the “sick role” and is subject to a longer period of being stressed by a serious diagnosis. Furthermore, an effective treatment must also be acceptable to the patient. For example, it is important to discuss a person’s values in relation to termination of pregnancy when offering prenatal screening for congenital disease for which termination is the only treatment.

When there is an effective treatment, screening can sometimes be an initiator of primary prevention. For instance, while treatment of HIV will not always eliminate the infection, screening followed by counselling and treatment may reduce transmission to others by modifying the patient’s behaviour and by reducing the patient’s infectivity by reducing the viral load.

The test

There should be a suitable test or examination

In principle, a screening test should be simple, quick, cheap, reliable and valid. In reality not all tests measure up. For instance, sigmoidoscopy and colonoscopy for colon cancer, mammography for breast cancer and cytology for cervical cancer require skilled professionals and sophisticated equipment. For this reason they can be costly. Blood pressure may be affected by a number of factors relating to the patient (e.g., white coat hypertension), the person doing the measurement (e.g., difficulty hearing the sounds) and the instrument (e.g. badly calibrated, badly positioned). In consequence, blood pressure measurement is not very reliable (see Chapter 6). Prostate specific antigen (PSA) is a marker of prostate cancer, but it can be elevated in benign prostatic disease. Furthermore, it cannot distinguish between cancers that will eventually kill and those that progress too slowly to cause problems. A valid test is one that measures what we want it to measure. In effect, although PSA is simple, quick, cheap and reliable, it is not a valid indicator of cancer that requires treatment.

The test should be acceptable to the population

High rates of adhesion to screening programmes are necessary to achieve appreciable change in population mortality. Promotion of screening, using a population approach or within the clinical setting, is a way to improve test acceptance. Promotion can act on a number of behavioural triggers (see Chapter 2) to increase participation in the screening programme. However, there are ethical issues involved. Patient autonomy may be compromised by the way that information is given. For instance, most information about breast cancer screening stresses the population benefit and not the benefit to the individual. Statements such as “Mammography screening will reduce your risk of death from breast cancer by 30%” confuse relative and absolute risk. The information that a patient needs is: “Mammography screening will reduce your risk of death from breast cancer from 4% to 3%.” Although the benefit for a whole population is considerable, the benefit for the individual can be fairly small (see Chapter 8).

The way information is received can also lead to unrealistic expectations from screening. One of the most compelling pro-screening arguments is the personal testimony of patients who, having been screened, were diagnosed and treated. These patients often attribute their continued survival to the fact of having been screened and sometimes become advocates for it. This may result from a need for cognitive consistency. If a patient is persuaded to be screened, he will unconsciously realign any previous anti-screening attitudes that he had in order to prevent cognitive dissonance. As a result, although the results of screening are well known and pamphlets clearly state that screening does not save every life, those who undergo screening tend to believe that theirs is the life that will be saved. However, for an individual case, it is impossible to tell if screening has made a difference. Sadly, even the most effective screening programmes are not 100% effective. For example, screening and treatment for colorectal cancer reduces mortality by about 15%, and people whose cancers were detected by screening can still die from them. Controlled trials show that, at best, breast cancer screening reduces mortality by 30%, not 100%. About 70% of unscreened women diagnosed with breast cancer do not die from the cancer.8

Even careful clinicians can have difficulty ensuring that patients have a realistic view of the risks and benefits of screening so that they can give properly informed consent (see Chapter 10 on communicating risks) Sending patients letters or other reminders improves screening uptake. However, it comes with the risk that patients perceive the reminders as having more authority than they do and, in consequence, abdicate their right to informed consent.13

Finally, deprived, vulnerable people are generally at greater risk of disease. They are also less likely to respond to promotion of screening than those with better health determinants. Screening activities that do not ensure equitable access can increase health inequities.

The health care system

There should be an agreed policy on whom to treat as patients

Few indicators of pre-clinical disease are dichotomous; that is, few differentiate sharply between who will develop the condition and who will not. Indeed progress in medical imaging and screening techniques means that disease can be identified their very early stages. Unfortunately, the natural history of these very early lesions is poorly understood. For instance, in cancer screening, it can be very difficult to distinguish between inflammation and early neoplasia. Although some findings on imaging or cytology can be categorized definitely as neoplastic or as benign, others are intermediate. Even experienced readers may have difficulty interpreting these. Blood sugar, cholesterol, and blood pressure are all continuous scales; there is no definite threshold for hypertension, hypercholesterolemia or hyperglycemia, there is only more or less risk of developing the associated diseases. A cutting-point must be set to distinguish those who are at higher risk of developing disease from those who are at average risk, as described in Chapter 6. This point is set by considering the Receiver Operating Characteristic (ROC) curve (see RECEIVER OPERATING CHARACTERISTIC in Glossary). If the cut-off point is set too far towards the high risk end of the scale, the screening is likely to be ineffective because too many patients with the disease will be missed. If the point is too far towards the low risk end of the scale, the costs of investigating and reassuring large numbers of patients at low risk will outweigh the benefits of evaluating and treating high-risk patients.

Because it is often hard to detect disease in its early stages and bearing in mind the potentially damaging consequences of an erroneous test result, only tests with excellent performance characteristics (sensitivity and specificity) should be used for screening. In order for mammography to be a suitable screening test, technicians and radiologists who carry out screening tests, even if they are experienced in diagnostic mammography, need training to ensure that the performance characteristics of their screening approach those in breast cancer screening trials.

Facilities for diagnosis and treatment should be available

This would seem an obvious criterion: if waiting lists are long or if facilities for follow up and treatment are not conveniently accessible, implementation of a screening programme should be reconsidered. Harm occurs when asymptomatic people are labelled as possibly having a disease but further investigation cannot be carried out. Harm also occurs when they are diagnosed and no treatment offered. When a region-wide screening programme is under consideration, planning for diagnosis and treatment is usually included. So, in developing the Canadian breast cancer screening system, provinces had to ensure that diagnostic facilities were prepared to deal with the added workload that systematic screening produced. Community clinics and primary care providers should consider the impact of their screening programmes on the local health system as well as the availability of specialist services. For instance, the treatment of childhood obesity requires a structured behavioural programme. A clinician who practices in an area where such a programme does not exist and who is not prepared to provide one should consider not systematically screening children for obesity.

The cost of case-finding (screening and the resulting diagnosis) and the treatment of the found cases should be economically balanced in relation to possible expenditure on medical care as a whole.

Economic costs

The cost of a single test may be low but the cost of screening a whole population can be considerable. Furthermore, patient management systems are needed for call, recall and follow up of results; systems are needed to ensure that screening equipment is maintained and that screening staff are adequately trained, and patient safety systems should be in place to reduce the risk of medical error. Systematic evaluation of screening processes and outcomes should also be in place.

The cost of investigating people with screen-detected abnormalities, many of which are false positives, and the cost of treating the true positive tests have also to be considered.  One comparison of the costs of breast cancer treatment with and without screening found that screening reduced the cost of treating breast cancer by 21%. At the same time, screening increased overall health care costs, including those not related to breast cancer, by nearly 6%.14  Clinicians should recognize that spending on a procedure that has not been shown to be beneficial reduces the resources available for effective procedures.


Other costs – Beneficence and non-maleficence

While Wilson and Jungner mention only economic costs, the ethical clinician needs to balance all likely costs against all likely benefits.  Apart from the cost to the healthcare system, screening incurs physical, mental, and financial costs to patients and their families, as summarized in Table 9.1.

The clinician who proposes interventions to healthy patients, particularly if they have not asked for them, has an added responsibility to ensure the benefit is likely to outweigh the harm. Screening often has little or no impact on an individual’s risk from the disease being screened for, while the test and follow-up procedures carry an inherent, although generally minimal, risk of harm.

Patients also ask for screening tests, such as total body scans, an annual physical examination or a PSA test, for which the evidence does not support their use in most populations. The physician must balance his duty not to harm against his duty in relation to patient autonomy of choice. There is no ready answer to this dilemma: physicians should educate their patients and negotiate with them bearing these concerns in mind.

Table 9.1: The physical and emotional costs of screening

Costs due to: Examples
The test itself Colonoscopy carries a small risk of bowel perforation; mammography exposes the breast to radiation, albeit minimal.
Procedures engendered by false positive results Diagnostic and treatment procedures carry risk. In the case of treatment for putative cancers, these can be severe. About 10% of women who undergo breast cancer screening for the first time and about 6% of women getting subsequent screening undergo further testing only to find that the screen result was falsely positive.15
Unnecessary anxiety after false positive results Most patients experience anxiety on being told of a positive screening result. Some patients take considerable time to get over the anxiety. As many as 5% of Pap tests in young women are false positives.
False reassurance after negative results After having a negative screen, whether a true or a false negative, some patients ignore subsequent symptoms of the disease.
Imposition of difficult choices Men over 65 found on screening to have an asymptomatic abdominal aortic aneurysm have a choice of either undergoing a very risky operation immediately or face a 70% risk of death within the next five years.
Prolongation of the period of illness As their disease is detected earlier, patients who are screened live longer with the diagnosis. In some cases, the consequent anxiety and changes to lifestyle reduce the quality of life.
Labelling If the disease is viewed negatively, the patient runs the risk of being viewed, or viewing himself, prejudicially. Consequences are both social and psychological. They can also be financial if the patient loses his job or can’t find insurance after being diagnosed.

 Screening should be a continuing process and not a “once and for all” project.

The screening interval (time between screens) should be based on the natural history of the disease – how long between the onset of screen detectable disease and irreversible damage – and the sensitivity of the test – at what stage can the test detect the disease. Too long an interval means that many cases will become symptomatic in the intervals between screenings, so the benefit of early detection is lost. Too short an interval can reduce the PPV as well as increasing the number of tests required to detect a case. Annual screening is often recommended, not on the basis of evidence, but for convenience. For instance, some health professionals still recommend annual Pap smears even though the best outcomes for cervical screening are from Finland and Holland, where the screening interval is five years but the programme reaches a very high proportion of the population. The common practice of screening compliant women every year ties up resources and turns attention away from groups most at risk from invasive cervical cancer and who are least likely to be screened, such as women living in rural areas, Aboriginal women, recent immigrants, older women, and those with lower incomes. This is an example of the inverse care law.

The inverse care law

An unfortunate inverse relationship between need and demand was termed “the inverse care law” by Julian Tudor Hart: those at highest need are the least likely to receive care particularly preventive care.16

An unintended consequence of interventions is often that their initial uptake occurs among groups such as opinion leaders, informed people and the “worried well” so that their health improves, widening the disparities in health between them and other, less informed people. If subsequent population health efforts succeed in increasing uptake by specifically targeting the less informed people, the gap can narrow again. However, as innovations are always being proposed, the health gap produced by delayed uptake of innovations is likely to persist. A Public Health Agency of Canada report presented a diagram illustrating how delayed adoption of innovations can generate socioeconomic disparities in health (see page 9, Figure 3 of the report).

Paul’s prostate

Having started to discuss prostate cancer screening, Dr. Rao realised that Paul had a number of deep-seated fears and false beliefs on the subject. Dr. Rao asked Paul to drop into Nurse Jennings’ office to get fact sheets on prostate cancer and prostate cancer screening. He also asked Paul to make another appointment when Dr. Rao would have more time to continue the discussion.

WHO synthesis of screening criteria17

Since the publication of the Wilson and Jungner criteria in 1968, some formal, organized screening programmes have been introduced, and others have been considered. Opinions on screening criteria have also progressed; the criteria now include the notion of screening programmes, informed choice for patients, equity in application of the programme, and evidence of costs and benefits, and of effectiveness of the programme.

  1. The screening programme should respond to a recognized need.
  2. The objectives of screening should be defined at the outset.
  3. There should be a defined target population.
  4. There should be scientific evidence of screening programme effectiveness.
  5. The programme should integrate education, testing, clinical services, and programme management.
  6. There should be quality assurance, with mechanisms to minimize potential risks of screening.
  7. The programme should ensure informed choice, confidentiality, and respect for autonomy.
  8. The programme should promote equity and access to screening for the entire target population.
  9. Programme evaluation should be planned from the outset.
  10. The overall benefits of screening should outweigh the harm

Self-test questions

1. One of your colleagues has just read an article that shows that screening for orange disease using the marmalade test can prolong survival of patients with the disease. What information do you need before deciding whether or not to support this conclusion?

You need to know what kind of evidence is presented in the article. An observational cohort study that showed that patients who were screened had better results that those who were not is subject to bias due to self-selection; patients who choose to have a test generally do better than those who don’t because they tend to also have other more positive health determinants. A case series that showed that patients presenting after screening had a longer survival time than those presenting when the disease is clinically apparent is subject to lead-time and length biases; screening picks up cases earlier on in the natural history of disease with the result that, as the time of diagnosis is advanced, the period during which the patient is known to have the disease is prolonged. This makes it appear that survival has been prolonged.  Furthermore, screening favours the diagnosis of indolent forms of disease, so cases picked up by screening are likely to be those which progress slowly.Because of such problems the only acceptable evidence for screening comes from a randomised controlled trial. High level evidence in screening is particularly important because screening always prolongs the period of illness and often brings psychological, social and financial problems to people who would otherwise have enjoyed apparently good general health for a longer period.

If you are satisfied that the article describes a valid controlled trial which shows that screening results in a reasonably large clinical benefit, you then need to check a few other factors before forming an opinion. The main one concerns how much work has already been done on the subject and is if the article agrees with it. It is generally rash to accept findings that disagree with those of other valid studies. It is also rash to act on the basis of a single trial.

You would also like to know background information about orange disease, about its frequency, its likely outcomes and the effectiveness of treatment options. Is the natural history known, and is early treatment more effective than treating at a later stage? Your assessment includes consideration of both length and quality of life, as well as the risks of treatment. Secondly, is a screening test possible – is there a preclinical phase that lends itself to early diagnosis?

Once you are satisfied that the evidence from a number of sources is solid, then you need to look at issues in implementation. (See the WHO criteria) You would like to know about costs of screening and of treatment for the patient and for the health care system, and whether or not you have at your disposal the means of carrying out the test, diagnosing patients who are found to be at high risk of orange disease and the means of treating those confirmed to have orange disease. You would like to know about the performance characteristics of the serum marmalade test: its positive and negative predictive values in the general population. You would also like to know whether or not patients find it acceptable.

 

Reflection Questions

1.   Describe the breast cancer screening programme in your province.
2.   What are your provincial guidelines for prostate cancer screening?
3.   At what level of glycemia or blood sugar do your professors recommend action should be taken in healthy patients?

References

  1. Wilson JMG, Jungner G. Principles and practice of screening for disease. Public Health Papers No 34. Geneva: World Health Organization; 1968. p. 164.
  2. Canadian Task Force on Preventive Health Care. About the CTFPHC 2016 [cited May, 2016]. Available from: http://canadiantaskforce.ca/about-us/about-the-ctfphc/.
  3. Porta M, editor. A dictionary of epidemiology. New York (NY): Oxford University Press; 2008.
  4. Leung GM, Lam TH, Thach TQ, Hedley AJ. Will screening mammography in the East do more harm than good? American Journal of Public Health. 2002;92(11):1841-6.
  5. Leung GM, Thach TQ, Chan E, Foo W, Meng O, Fielding R, et al. Short-term, medium-term, long-term, and lifetime risks of developing and dying of breast carcinoma in a Westernized Chinese population: Evidence from Hong Kong between 1976 and 2000. Cancer. 2005;103(3):501-8.
  6. Holowaty P, Miller AB, Rohan T, To T. Natural history of dysplasia of the uterine cervix. Journal of the National Cancer Institute. 1999;91(3):252-8.
  7. Thompson IM, Ankerst DP. Prostate-specific antigen in the early detection of prostate cancer. Canadian Medical Association Journal. 2007;176(13):1853-58.
  8. Morrison AS, Brisson J, Khalid N. Breast cancer incidence and mortality in the breast cancer detection demonstration project. Journal of the National Cancer Institute. 1988;80(19):1540-7.
  9. Viera AJ. Predisease: when does it make sense? Epidemiologic Reviews. 2011;33:122-34.
  10. Canadian Diabetes Association. Clinical practice guidelines 2013 [cited 2016 November]. Available from: http://guidelines.diabetes.ca/?_ga=1.41276011.839397301.1478810223.
  11. World Health Organization. Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia 2006. Available from: http://www.who.int/diabetes/publications/diagnosis_diabetes2006/en/.
  12. Yudkin JS, Montori MV. The epidemic of pre-diabetes: the medicine and the politics. BMJ. 2014;349:g4485.
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