Principles Of Screening

Screening may be defined as the examination of apparently well or asymptomatic people in order to find out if they are likely or unlikely to suffer from disease. They can then, if they are likely to have the disease, be placed under treatment early in the natural course of the disease. The goal of screening is to detect and treat the disease as early as possible and thereby reverse or retard the disease process. Sometimes the object of the screening procedure is to find people at high risk of getting a disease. By identifying precursors of disease and correcting these, the disease may be postponed or at best prevented. There is no sharp line between a risk factor and a disease (2).

The screening procedure must always be followed by a treatment programme offering treatment to those found to have a disease or an increased risk of getting the disease. A screening programme can thus be divided into a diagnostic and a therapeutic component. In 1968, an increased interest in screening inspired the WHO to publish a Public Health Paper with the title 'Principles and practice of screening for disease' (3). This presented basic principles of screening together with practical and ethical considerations (Table 5.1). Launching a screening programme is a complex task, which if not done appropriately may lead to serious consequences. Several questions of an ethical and practical nature must be considered. By using certain screening principles, the chance of success will increase and the risk of serious adverse consequences will diminish.

The Importance of the Problem

The importance of the health problem needs to be regarded from the point of view of the individual as well as of the community. An uncommon condition with serious consequences for the individual, such

International Textbook of Obesity. Edited by Per Bjorntorp. © 2001 John Wiley & Sons, Ltd.

Table 5.1 Principles of screening

1. The condition sought should be an important health problem

2. There should be an accepted treatment for patients with recognized disease (condition)

3. Facilities for diagnosis and treatment should be available

4. There should be a recognizable latent or early symptomatic stage

5. There should be a suitable test

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

Modified from Wilson and Jungner (3).

as phenylketonuria, may warrant screening measures just as much as a more common but individually less serious condition.

The Necessity of an Accepted Treatment and Resources for its Implementation

Perhaps the most important criterion for a screening programme is to have an accepted treatment for those screened positive. It must be admitted that there are many difficulties in evaluating the outcome of a screening programme. Often, months or years must pass before the gains may be measurable. It is vitally important to determine whether earlier treatment really does give a better prognosis. If this is not the case, then there is no advantage in alerting the patient of the risk condition by early detection. No screening programme should be implemented without first having ensured that there are adequate resources in personnel and equipment for all individuals in need of treatment to get it. The responsibility is clearly on the health care organization that has initiated the screening examination and urged the individual to respond.

A Latent or Early Symptomatic Stage of the Disease

The disease must have a recognizable latent or early stage that can be detected by the screening test. The interval from this detection to the time when diagnosis ordinarily would have been made due to symptoms, which would make the person seek medical attention, is known as the lead time. In other words, lead time is the amount of time by which treatment may be begun earlier due to the screening programme. To summarize, if early treatment is not especially helpful, then there is no point in early detection.

Characteristics of the Screening Test

The validity of a screening test is measured by its sensitivity and specificity. 'Sensitivity' is the ability of the test to classify people as positive when they have the disease or the risk factor under study. 'Specificity' is defined as the ability of the test to classify people as negative when they do not have the disease or the risk factor under study. Having or not having the disease or risk factor in question should ideally be determined by a test that is a part of the diagnostic procedure. A diagnostic test may take more time and be more expensive to perform, and may have a lower degree of safety, but it is essential that it has the highest degree of accuracy. Consequently, an estimate of sensitivity should be regarded as the sensitivity of one test (the screening test) relative to another (the diagnostic test). The same kind of argument may be applied to the use of specificity. In reality, it is often impossible to use the diagnostic test on all screenees to verify the result of the screening test. For instance, the diagnostic test in cancer screening might be extensive surgery.

False positives are people who tested positive but do not have the disease or risk factor under study and false negatives are people who tested negative but do show the disease or risk factor under study. The 'positive predictive value' is the ability of the screening test to predict that there is a state of early disease or a high risk. The positive predictive value will depend in part on the screening test characteristics (sensitivity and specificity) and in part on the prevalence of the disease or risk situation. The reliability of the test is its capacity to give the same result on repeated measurements. Ideally, a screening test should be valid, safe, simple to perform, acceptable to the subject and inexpensive. The sensitivity of the test may be increased or decreased by changing the level at which the test is considered positive. An increase in the sensitivity will decrease the specificity and vice versa (Figure 5.1).

Figure 5.1 Schematic illustration of the relationship between different cut-off points and sensitivity and specificity of defining high-risk individuals and normals. Sensitivity is represented by the solid line and specificity by the dotted line

Adequate Understanding of the Natural History of the Target Disease

Whether a screening programme will be useful to a given population or not depends to a large extent on the natural history of the target disease. A main question is: Does early detection and treatment affect the course and prognosis of the disease? In order to answer that question well-planned and preferably randomized experimental studies must be conducted. In many cases, these clinical trials must be started as soon as possible. If not carried out speedily enough, the time trend may change the practice of treatment and treating the latent stage of the disease may be regarded as normal practice. In such a situation, it would no longer be considered ethical to perform a randomized clinical trial. As a result of this, scientific evidence, showing that the early detection and treatment really changes the natural course of the disease and improves its prognosis, may be lacking.

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