It must be very clear that even if prognosis is improved by early treatment, screening is only worthwhile if a satisfactory diagnostic test is available. This test must fulfil the following criteria:
·Detect cases in sufficient numbers
A screening test must be inexpensive and easy to perform and therefore is not usually the most valid diagnostic method for a disease.
In screening, therefore, it has to be accepted that some cases will remain undetected. The predictive value of a positive result is the probability that a person who reacts positively to the test actually has the disease.
As mentioned previously predictive value varies with the number in the population who have the disease to whom the test is applied. When this is low, then there are more false positive results than true positives, and predictive value falls.
It thus follows that even though a test functions well in normal clinical practice, it may not necessarily be useful for screening purposes. Sputum cytology has quite a high positive predictive value for bronchial carcinoma in patients presenting with haemoptysis, but if it is used to screen asymptomatic people, most positive results will be false.
·Can be performed at acceptable cost
·Must not carry side-effects that outweigh the benefits of screening
The average benefit to the individual from a screening programme is usually much smaller than if these same tests were performed for diagnostic purposes performed in response to symptoms, which necessitates that screening tests need to be safer than those used in normal clinical practice.
For example, even though the radiation dose from a chest x-ray examination is small, if the investigation forms part of a screening programme for tuberculosis, then even the very small risk of complication may outweigh the benefits of early diagnosis.
When the prevalence of pulmonary tuberculosis in the general population was very high, mass radiographic screening was justifiable. But it may no longer be so.
·Diagnosis and treatment
Once there is a positive result, there are additional requirements for follow-up of the screening results since the test only identifies those at risk of disease. Confirmatory or diagnostic testing is required, and if positive, an accepted treatment that can alter the course of disease must be available.
The corollary being there is no point offering screening, even with a very good test, when there is no treatment available for the condition detected.
The facilities and resources for these follow-up tests and treatments should be readily available. It is imperative that there is a consensus with a clear agreed policy among those doing the follow-up as to what should be done. Otherwise it would be impossible to determine outcomes and validate the usefulness of screening.
This is where the axe usually falls on most screening programmes since they are very expensive to run. They are resource intensive not only for the screening test itself, but also for follow-up and treatment.
These investments have to be made as a screening programme is being developed, whilst the benefits do not become available until some time in the future. Sufficient resources must be available to run the programme. This would involve not just equipment, but personnel as well as infrastructure to organise and manage the screening programme.
This will usually extend beyond the discipline in question. For example, in mammography, there will be an increased need for pathology services followed by the need for localisation and surgical treatment. All these put a severe strain on the nations resources.
It is often believed that screening programmes will save money. However, most economic analyses of screening programmes show that the savings do not offset the expenses.
At the same time, many screening programmes can represent an efficient investment of resources, when compared with other medical interventions.
It is generally accepted that lung cancers detected at an early stage in their development are more likely to be surgically resectable.
However, a large study in the United States failed to demonstrate any clear reduction in mortality from lung cancer among asymptomatic heavy smokers who were offered four monthly screening by radiography and sputum cytology, despite the fact that more resectable tumours were detected in the screened population.
The outcome of screening must be judged in terms of its effect on mortality or illness, and not simply by the number and severity of cases identified.
Survival might be longer in those screened, not because early treatment is beneficial, but simply because their tumours are being diagnosed earlier in the natural history of their disease.
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