15 - Screening

Question 1

Screening definition

Answer 1

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

Question 2

Validity

Answer 2

ability to distinguish between subjects with the condition and those without

Question 3

Gold Standard

Answer 3

definitive test used to assess the validity of a screening test where the true disease status of the individuals must be known

Question 4

Sensitivity

Answer 4

ability of the test to correctly identify people with the
disease
sensitivity = a ÷ (a+c)

Question 5

Specificity

Answer 5

ability of the test to correctly identify people without the
disease
specificity = d ÷ (b+d)

Question 6

Positive Predictive Value

Answer 6

likelihood that a patient with a
positive test result will actually have the disease
positive predictive value = a ÷ (a+b)

Question 7

Negative Predictive Value

Answer 7

likelihood that a patient with a
negative test result will not have the disease
negative predictive value = d ÷ (c+d)

Question 8

What is the predictive value of a test dependent on?

Answer 8

sensitivity and specificity AND the prevalence of the condition in the population

Question 9

Receiver Operator Characteristics (ROC) curves

Answer 9

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.

Question 10

Mass Screening

Answer 10

screening involving the whole population

Question 11

Targeted Screening

Answer 11

screening involving selected groups who are anticipated to have an increased prevalence of the condition

Question 12

Systematic Programme

Answer 12

where people are called for screening (e.g. cervical cancer, breast cancer)

Question 13

Opportunistic Programme

Answer 13

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).

Question 14

Major Screening Programmes in the UK

Answer 14

antenatal, neonatal and childhood, cancers, cardiovascular disease, infections

Question 15

Evaluation of a potential screening programme involves consideration of 3 main issues:

Answer 15

feasibility
effectiveness
cost

Question 16

Feasibility

Answer 16

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

Question 17

Effectiveness

Answer 17

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

Question 18

Types of Biases involved in Effectiveness

Answer 18

selection bias
Lead time bias
Length bias

Question 19

Cost

Answer 19

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.

Question 20

Ethics of Screening

Answer 20

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

Question 21

Prognosis

Answer 21

outcome of an illness, including duration of disease, mortality and morbidity