Population Based Screening Flashcards
What is screening?
A systematic attempt to detect an unrecognised condition using tests, examination or other provedures
Distinguishing between people who probably have a disease (or precursor) and those who probably do not
What are the 5 criteria used by the national screening committee to decide if a screening programme should be undertaken?
- Condition
- Test
- Intervention
- Screening Programme
- Implementation
For a screening programme to develop, what must we know about the condition?
Important health problem (variable frequency/severity)
Epidemiology
Natural history and disease progression
For a screening programme to develop, what factors must be considered regaring the test to be performed?
Complexity
Safety
Precise & Validated
Agreed cut off level
Agreed policy for further investigation
What is a key consideration when considering the proposed intervention following a screening programme?
Evidence that invervention leads to better outcomes at a pre-symptomatic phase
Clear policies as to who will be offered interventions and those who will not
What genereal principles of the screening programme must be clear to the national committee when deciding to develop a new screening programme?
Proven effectiveness in reducing morbidity or mortality (RCT data)
Benefit gained by individual outweighs any harms (risk of false positives)
Opportunuty cost should be balanced against providing healthcare elsewhere
Following implementation of screening programme, what must occur to ensure it is fit for purpose?
Quality Assurance
Information available to patinets (Informed Choice)
Adeuate staffing and facilities
What are the 4 features of test validity?
Sensitivity
Specificity
Positive Predictive Value
Negative Predictive Value
Define sensitivity
The proportion of people with the disease who test positive
(a.k.a. detection rate)
What is the formula used to calculate sensitivity?
True Positives
_____________________________
True Positives + False Negatives
Define specificity
Proportion of the people without the disease who are test negative
What is the formula used to calculate specificity?
True Negatives
_____________________________
True Negatives + False Positives
Define positive predictive value
Probability that someone who has tested positive, actually has the disease
What strongly influenes the positive predictive value?
Prevalece of the disease
What is the formula used to calculate the positive predictive value?
True Positives
_____________________________
True Positives + False Positives
Define negative preductive value
Proportion of the people who are tested negative who actually do not have the disease
What is the formula used to calculate the negative predictive value?
True Negatives
_____________________________
True Negatives + False Negatives
What are the implications of a false positive result?
Patients offered invasive testing which they do not need
People becomes patients when they are not ill
Unnecessary anxiety
Risk of lower uptake of screening in future
What are the implications of a false negative result?
Affected patients not offered diagnostic testing
False reassurance - may persent late with symptoms
What is lead time bias?
Screened patients appear to survive longer
Just diagnosed earlier!
What is length time bias?
False conclusion that screening is beneficial but actually:
Screening detects slow growing, less aggressive cases
Diseases detectable through screening may have more favourable outcome (may never have caused a problem)
What is selection bias in the context of screening programmes?
‘Healthy Volunteer’ Effect
Those engaging in screening programmes, often also engage in other health promotion activities e.g. diet/exercise
What are some negative views of screening programmes held by the public?
Morally obliged to take part - removes automony/choice
Social control - constant surveillance
