15 - Screening Flashcards
Screening definition
practice of investigating apparently healthy individuals to detect unrecognised disease or its precursors so that measures can be taken to prevent or delay the development of disease or improve prognosis
Validity
ability to distinguish between subjects with the condition and those without
Gold Standard
definitive test used to assess the validity of a screening test where the true disease status of the individuals must be known
Sensitivity
ability of the test to correctly identify people with the
disease
sensitivity = a ÷ (a+c)
Specificity
ability of the test to correctly identify people without the
disease
specificity = d ÷ (b+d)
Positive Predictive Value
likelihood that a patient with a
positive test result will actually have the disease
positive predictive value = a ÷ (a+b)
Negative Predictive Value
likelihood that a patient with a
negative test result will not have the disease
negative predictive value = d ÷ (c+d)
What is the predictive value of a test dependent on?
sensitivity and specificity AND the prevalence of the condition in the population
Receiver Operator Characteristics (ROC) curves
used to determine a cut- off value for a diagnostic or screening test. In order to set the cut-off value for a continuous variable, proportion of true-positives and false-positives are calculated for possible values. The ROC curve is a graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values. The choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity. Generally, there is a trade-off between sensitivity and specificity, and the decision must be based on their relative importance.
Mass Screening
screening involving the whole population
Targeted Screening
screening involving selected groups who are anticipated to have an increased prevalence of the condition
Systematic Programme
where people are called for screening (e.g. cervical cancer, breast cancer)
Opportunistic Programme
when a person presents to the doctor for some other reason and they are offered a test (e.g. Chlamydia screening in young people, blood pressure screening in older people).
Major Screening Programmes in the UK
antenatal, neonatal and childhood, cancers, cardiovascular disease, infections
Evaluation of a potential screening programme involves consideration of 3 main issues:
feasibility
effectiveness
cost
Feasibility
depend on how easy it is to organise the population to attend for screening, whether the screening test is acceptable, whether facilities and resources exist to carry out the necessary diagnostic tests following screening
Effectiveness
evaluated by measuring the extent to which implementing a screening programme affects the subsequent outcomes. This is difficult to measure because of a number of biases that affect most of the study designs used
Types of Biases involved in Effectiveness
selection bias
Lead time bias
Length bias
Cost
Resources for health care are limited and there are many competing demands for available money, health care professionals and facilities.
The relative cost-effectiveness of a screening programme compared with other forms of health care should therefore be considered.
Costs relate not just to the implementation of the screening programme but also to the further diagnostic tests and the subsequent cost of treatment.
On the other hand, in the absence of screening, costs will be incurred by the treatment of patients in more advances stages of disease.
Ethics of Screening
There may be a risk attached to the screening test or subsequent diagnostic test
A false positive result can cause unnecessary anxiety
There may be other unplanned effects of a positive test A false negative result will give false reassurance
Prognosis
outcome of an illness, including duration of disease, mortality and morbidity
