Screening Programmes Flashcards
What is screening?
Screening is the examination of asymptomatic people to identify early disease or the precursors of disease with the usual aim of preventing or reducing mortality or morbidity. Screening can be defined as ‘The systematic application of a test or inquiry, to identify individuals at sufficient risk of a specific disorder to warrant further investigation or direct preventive action, amongst persons who have not sought medical attention on account of symptoms of that disorder.’ It can be directed at the whole population (mass screening) or at specific groups of people considered to be at high risk of developing the disease (selective screening). Whilst screening is usually undertaken to reduce mortality and morbidity, through effective, early treatment of identified cases of disease, some screening programmes are undertaken for slightly different reasons. For example, antenatal screening for Downs Syndrome or Spina Bifida, where the purpose of screening is to allow parents the possibility of terminating the pregnancy (and thereby preventing future morbidity). In screening for communicable disease such as HIV, the aim is not only to reduce mortality and morbidity of the person who is HIV positive, but also to prevent the mortality or morbidity of their at risk contacts.
Screening can also be done pro-actively or opportunistically. In pro-active screening members of a target population are invited to attend for testing in a systematic programme which will cover the whole of that population over a defined period of time. For example, women between 20 and 64 are targeted for regular cervical screening test every 3 to 5 years. Opportunistic screening occurs when a test for an unsuspected disorder at a time when a person presents to the doctor for another reason. For example, most people have blood pressure check when they visit their GP for any medical concern and this could identify people at the risk of heart disease.
Much attention is focused on screening tests, for example the cervical smear test or blood pressure measurement, but it is inappropriate to think of screening as simply a test. A screening programme consists of all those activities from the identification of the population likely to benefit right through to definitive diagnosis and treatment.
There is sometimes confusion over the terminology used in screening and clinical case finding. Identification of people with a specific disease in clinical practice or research is sometimes referred to as screening (for example, screening for depression). This is misleading because case finding relies on the identification of people with known disease whereas the aim of screening is to identify people in the early stages (pre-clinical phase) of disease before they are aware that they have the disease.
How can screening be defined?
‘The systematic application of a test or inquiry, to identify individuals at sufficient risk of a specific disorder to warrant further investigation or direct preventive action, amongst persons who have not sought medical attention on account of symptoms of that disorder.’
What is screening directed at a whole population called?
Mass screening (e.g. PKU)
What is screening directed at specific groups of people considered to be at high risk of developing the disease called?
Selective screening (e.g. Coal miners)
What is the purpose of screening?
Whilst screening is usually undertaken to reduce mortality and morbidity, through effective, early treatment of identified cases of disease, some screening programmes are undertaken for slightly different reasons. For example, antenatal screening for Downs Syndrome or Spina Bifida, where the purpose of screening is to allow parents the possibility of terminating the pregnancy (and thereby preventing future morbidity). In screening for communicable disease such as HIV, the aim is not only to reduce mortality and morbidity of the person who is HIV positive, but also to prevent the mortality or morbidity of their at risk contacts.
What is proactive screening?
In pro-active screening members of a target population are invited to attend for testing in a systematic programme which will cover the whole of that population over a defined period of time. For example, women between 20 and 64 are targeted for regular cervical screening test every 3 to 5 years.
What is opportunistic screening?
Opportunistic screening occurs when a test for an unsuspected disorder at a time when a person presents to the doctor for another reason. For example, most people have blood pressure check when they visit their GP for any medical concern and this could identify people at the risk of heart disease.
Is a screening programme simply a test?
Much attention is focused on screening tests, for example the cervical smear test or blood pressure measurement, but it is inappropriate to think of screening as simply a test. A screening programme consists of all those activities from the identification of the population likely to benefit right through to definitive diagnosis and treatment.
What criteria are used to assess whether certain diseases are suitable for screening?
Not all diseases are appropriate for screening. General criteria for screening have been proposed by the World Health Organisation (WHO) and expanded to account for current available evidence and concerns about the adverse effects of health care.
The criteria for appraising the viability, effectiveness and appropriateness of a screening programme include assessment of the condition, the test, the treatment, and the screening programme.
Describe the characteristics of a condition that may be suitable for inclusion in a screening programme.
1) . The condition should be an important health problem.
2) . The epidemiology and natural history of the condition, including development from latent to declared disease, should be adequately understood and there should be a detectable risk factor, disease marker, latent period or early symptomatic stage.
3). All the cost-effective primary prevention interventions should have been implemented as far as practicable.
The disease should be an important health problem. That is, it should be fairly prevalent and result in significant mortality or morbidity if untreated. Population screening for rare disease is not efficient but is sometimes undertaken (for example in phenylketonuria), when there is a good screening test available that is cheap, easy to administer, is not associated with significant risk and where the health consequences of non-treatment in the early stages of the disease are serious.
The natural history of the disease should be well understood. Many screening tests detect a range of abnormalities from slightly to severely abnormal. Some knowledge of the prognosis of different grades of abnormality is needed to inform decisions about when to offer treatment. In many cases there is poor understanding of the natural history of disease. The assumption is that the disease will inevitably progress to a stage where it causes mortality of significant morbidity. However, this is not always the case. Even in serious disease some cases either spontaneously resolve or fail to progress and this may happen in the pre-clinical phase. In other words, without screening, such people would be completely unaware that they had ever had the disease. Even within the same disease, there is often considerable variability in the rates of progression ease in different people. For example, in lung cancer, the size of the tumour does not necessarily correlate with prognosis. Patients with 3cm masses have the same outcomes as those with masses less than 1cm.
The disease must pass through a detectable pre-clinical phase. Since the purpose of screening is the detection and early treatment of disease, there is little point in screening for a disease that cannot be detected before the onset of symptoms as this would not result in earlier treatment.
Primary prevention of the disease should be implemented. If there is any primary prevention of disease, this would be important to have in place to prevent disease before it occurs.
Describe the characteristics for a test that may be suitable for inclusion in a screening programme.
1) . There should be a simple, safe, precise and validated screening test. Must have high sensitivity and high specificity. Sensitivity should be almost perfect but also want specificity to be high.
2) . Need t understand the range of test values we might get in the population. The distribution of test values in the target population should be known and a suitable cutoff level defined and agreed.
3) . The test should be acceptable to the population you are screening and to the health care service providing these tests - e.g. shouldn’t produce too many false positives, in genetic testing we should consider whether the presence of the variant means a person will actually develop the disease etc.
4) . There should be an agreed policy on the further diagnostic investigation of individuals with a positive test result and on the choices available to those individuals.
Describe the characteristics for a treatment that may be suitable for inclusion in a screening programme.
1) . There should be an effective treatment or intervention for patients identified through early detection, with evidence of early treatment leading to better outcomes than late treatment.
2) . There should be agreed evidence based policies covering which individuals should be offered treatment and the appropriate treatment to be offered. Clinical management of the condition and patient outcomes should be optimised by all health care providers prior to participation in a screening programme.
There must be effective treatment available for the disease. It would be unethical and a waste of resources to identify people with pre-clinical disease for which there was no effective treatment. This has been one of the dilemmas in screening for prostate cancer where the treatment most frequently offered to men positively identified as having prostate cancer is invasive surgery which is associated with a high risk of incontinence and impotence, despite the lack of evidence that this is more effective than ‘watchful waiting’ (i.e. no intervention) in most men.
Assuming the treatment is effective, early treatment must offer some advantage over later treatment. Screening is based upon the assumption that treatment in the early stages of the disease will be more effective in reducing/preventing mortality and morbidity than treatment when the disease is more established. For example, in most cases of cancer, it is accepted that early treatment of smaller, localised tumours is more likely to reduce mortality than treatment of metastatic disease.
Describe the evaluations that should be made of a screening programme.
1) . There should be high quality evidence from Randomised Controlled Trials that the screening programme is effective in reducing mortality or morbidity.
2) . There should be evidence that the complete screening programme (test, diagnostic procedures, treatment/ intervention) is clinically, socially and ethically acceptable to health professionals and the public.
3) . The benefit from the screening programme should outweigh the physical and psychological harm (caused by the test, diagnostic procedures and treatment).
4) . The opportunity cost of the screening programme (including testing, diagnosis and treatment) should be economically balanced in relation to expenditure on medical care as a whole.
5) . There should be a plan for managing and monitoring the screening programme and an agreed set of quality assurance standards.
6) . Adequate staffing and facilities for testing, diagnosis, treatment and programme management should be available prior to the commencement of the screening programme.
7) . All other options for managing the condition should have been considered (e.g. improving treatment, providing other services).
Describe the characteristics of screening tests.
The purpose of screening is to detect pre-clinical disease i.e. disease that is asymptomatic. Most screening tests, therefore are physiological, biochemical or anatomical and include x-rays, CT scans, mammography, cervical cytology, measures of blood pressure and so on. Suitable screening tests should be acceptable to the target population (otherwise there will be poor uptake), inexpensive, easy to use and interpret, safe, reliable, valid and have good sensitivity and specificity.
Describe possible sources of error in screening tests.
1) . Inherent error (e.g. x-ray resolution)
2) . Application error (x-ray wrong exposure)
3) . Interpretations error (x-ray reading)
There are several sources of error in screening tests, some of which are inherent in the tests themselves and others in their application and interpretation. For example, x-rays may be badly performed or not taken in a standardised way, smears may fail to capture the malignant cells that were present, some biochemical markers may be very unstable and the results of all tests are potentially open to misinterpretation or misclassification. In screening, the sensitivity and specificity of the tests are of particular importance.
What is meant by the sensitivity of a test?
Sensitivity of the test is the probability that it will correctly classify people with pre-clinical disease as positive (i.e. the proportion of true positives correctly identified) or the sensitivity of the test is the proportion of individuals classed as positives by the gold standard who are correctly identified by the study test. A high sensitivity means the test is correctly identifying a high proportion of the true positives (ie true exposed or true diseased).
What is meant by the specificity of a test?
Specificity is the probability that the test will classify people who are not diseased as negative (i.e. the proportion of true negatives correctly identified) or the specificity of the test is the proportion of individuals classed as negatives by the gold standard who are correctly identified by the study test. A high specificity means the test is giving few positive results in the unexposed or non-diseased. For the test to be valid, a high sensitivity and a high specificity are needed. The use of sensitivity and specificity can be used for screening as well as questionnaire as shown below.
What is the easiest way to calculate values for sensitivity and specificity?
The easiest way to calculate these values is to put the numbers in a cross tabulation of a reference (gold standard) test and our screening test.
True positives = a
False positives = b
False negatives = c
True negatives = d
Sensitivity = a/(a+c)
Specificity = d/(b+d)
True prevalence = (a+c)/(a+b+c+d)
Why do we need a good specificity in a screening test?
A good specificity means few false positive. This means there will be few false positives and less people worried unnecessarily.
