Screening Flashcards
Screening
investigating apparently healthy individuals to detect unrecognised disease/ precursors
Why perform screening?
so measures can be taken to prevent/ delay development of disease/ improve prognosis.
If earlier detection does not offer any hope of improved outcome
screening generally not indicated
Screening may be used for risk factors
identify people at higher risk of developing disease where interventions will reduce risk
Screening may also be used to identify people with infectious disease
Treatment/ control measures improve outcome for individual /prevent ongoing transmission
How could screening do more harm than good?
false alarms,
inducing anxiety
treatment of early disease which would not otherwise have become a problem
Ideal screening test
simple safe acceptable inexpensive repeatable valid (sensitive & specific).
Screening tests are not the same as diagnostic tests
Aims to identify people with precursors/ at risk of condition.
Further diagnostic tests performed to confirm diagnosis
Validity
ability to distinguish between subjects with condition & those without
To assess validity of a screening test, true disease status of individuals must be known, usually through a definitive test which is referred to as
Gold Standard
Sensitivity
Ability of test to correctly identify people with disease
Sensitivity equation
Positive diseased / All diseased results
Specificity
ability of test to correctly identify people without disease
Specificity equation
Negative non diseased/ All non diseased results
Positive predictive value (PPV)
likelihood that a patient with a positive test result will actually have the disease
PPV Equation
Positive diseased/ All positive results
Negative predictive value (NPV)
likelihood that a patient with a negative test result will not have the disease
NPV Equation
Negative non diseased/ All negative results
Receiver Operator Characteristics (ROC) curves
used to determine a cut- off value for a diagnostic or screening test
Mass screening
Screening involving the whole population
Targeted screening
Screening of selected groups who are anticipated to have an increased prevalence of the condition
Systematic screening
People are called
Opportunistic screening
Patient presents for another reason and are offered a test
Major screening programmes in the UK
Antenatal Neonatal Childhood Cancers Infections Cardiovascular disease
Evaluating screening programmes
Feasibility
Effectiveness
Cost
Feasibility of screening
ease of organising population to attend
whether screening test is acceptable,
whether facilities and resources exist to carry out necessary diagnostic tests following screening.
Effectiveness of screening
measuring extent to which implementing a screening programme affects subsequent outcomes
Why is it difficult to measure effectiveness of screening?
Selection bias- those who participate
Lead time bias- screening identifies disease that would be identified at a later stage (apparent increased survival length)
Why is cost of screening important?
Resources for health care are limited
Many competing demands for available money, health care professionals and facilities
Ethics of screening must be considered
may be a risk attached to the screening test or subsequent diagnostic test
Cancer screening UK
Cervical
Breast
Bowel
2 rogue screening programmes UK
Abdominal aortic aneurysm
Diabetic eye screening
